Here are some random tips and thoughts on recording acoustic guitars taken from several of my postings to the newsgroups.
-- The most important part of getting a good sound is the source itself. If you have a cheap acoustic guitar that sounds poor by itself, it certainly isn't going to sound any better on a recording. Any instrument that sounds really good by itself should be fairly easy to capture in a recording, assuming your equipment is not doing something to screw it up.
-- Where you are recording can have a big impact as well. The sound of the room will be captured along with the performance, no matter how close you place the microphone to the source. If you are close to a reflective surface (a wall, for example), the reflections from that wall may actually cause some weird phases anomalies with the direct sound at the microphone, and can do weird things to the sound. Try moving the performers and the microphone around a bit and experiment to find the best sounding position for your recording environment.
-- Microphone placement can make a big difference as well. A one microphone technique that is pretty common is to use a small diaphragm condenser (such as an AKG 451) and position it so that it is roughly pointing at the junction of the neck and the body of the guitar, and angled more towards the body. You can get in pretty close if you want that closer, brighter sound, or you can move back and even up (then angled down towards the guitar) from there until you find the sound you are after. The closer to the body, the warmer the tone (typically), while the more you move up the neck generally will make it brighter. Usually pointing right at the sound hole is a bad idea because it will be too boomy. If you also have a large diaphragm condenser and want to go for kind of a natural stereo image, I've had good luck by using the above technique for the small microphone, and then placing the large microphone near the other side of the guitar, aimed roughly at the bridge... again moving it around to find the sweet spot for that guitar. I've also had good luck in the past using a couple of small diaphragm condenser (such as KM84s) set two or three feet in front of the guitar, centered at the sound hole, but in an X-Y type of configuration (so that neither mic is actually pointed right at the sound hole)... this gives a different kind of natural stereo image and works even better when you are in a really nice sounding room. Also, no matter which technique you use, experiment quite a bit with the distance between the microphone(s) and the guitar, as just an inch or two can really change the sound.
Those are a few general tips that are applicable to any microphone setup. As far as specific microphones for acoustic guitars, I like to use small diaphragm condenser microphones when doing the one microphone technique. Some of the Audio Technica small diaphragm condenser microphones, in the AT 40xx series, are very nice for acoustic guitar and don't cost as much as the studio standard AKG 451 .... or, if you have a slightly larger budget, the Neumann KM184 microphone would be an excellent choice as well. For large diaphragm condenser microphones, the Neumann TLM 103 or TLM 193 are great microphones that work for a wide variety of situations, and cost quite a bit less than the higher end Neumann microphones. On a lower budget, the Rode NT1 is a pretty good sounding large diaphragm condenser microphone.
When I was looking for an acoustic guitar microphone for my project studio, I was pretty much used to using AKG 451s and Neumann KM84s at the big studio. But I needed something easier to find and easier on the wallet, and someone suggested the AT 4031 microphone. AT has some higher priced ones, but the person I was dealing with thought that the 4031 was the best buy for the money and would be the closest sounding to the 451. I worked on an acoustic project with a friend of mine and we tracked guitars in the hallway of the home I was in then with the AT 4031 running through a Drawmer 1960. We also tracked some guitars for the same project out at the big studio I worked at with an AKG 451 going through some really nice pre-amps and either an LA2A or 1176. When we compared the tracks, the guitar player liked the sound I got with the AT 4031 much better than what we did at the big studio. The AT 4031 is naturally a bit warmer sounding than the AKG 451, which tends to be a bit brighter (which I usually like when I'm doing rock/pop stuff and there are many other things in the mix). For the solo ac gtr stuff, the sound of the AT 4031 was the winner for that particular project, with that guy's particular guitar. All the tracks were very nice sounding, but he just favored the sound of the AT4031 a bit more than the 451.
As far as pre-amps go, get as nice of a sounding microphone pre-amp as you can afford, preferable one with a built-in compressor as well to do double duty on the way into your computer. I use to have both a TL Audio dual tube pre-amp/compressor, and a Drawmer 1960 tube pre-amp/compressor that I used in my home studio, and they both had their purposes. Recently, I upgraded my signal path to even higher end components and now use a Great River ME-1NV as my main pre-amp which I then usually run through a Cranesong Trakker compressor. Those two devices combined were around $3000 when I bought them, but they both are highly adjustable and I can get a wide range of tones from them.
On the lower end of the scale, many people like those little ART Tube MPs. Very inexpensive little tube microphone pre-amps that sound pretty decent for the money, from what I've heard. However, for my "lower end" pre-amp and compressor combo I went with the FMR Audio RNP and RNC rack together in one of the nice Funk Logic rack trays. The RNC is only around $200 and the RNP is around $450, but they easily compete against pre-amps and compressors in the $1000 to $2000 range.
Another common weak link of an audio chain are the A/D converters for getting the audio into the digital world. The cheap converters on the older Sound Blaster type cards simply won't cut it. Some of the newer SB Live cards will sound better, but still would not be my first choice. I would get a serious pro sound card, preferably one where the interface is away from the computer, or, better yet, get a card that has digital inputs and then buy a decent set of outboard A/D converters such as those made by Apogee (the Rosetta, for example) or Lucid. Some of these are useful, and will make a difference when digitally. If you get better sounding converters with the appropriate digital outputs to connect to your particular recording device, you can definitely make a difference in the sound quality.
I think that most of the time, the people posting to news group are more interested in the technical side of how to capture a sound, and most of us just take for granted that they are starting with a great sounding instrument with a great player and just need to know the "best" way to capture that. I try to point out as often as possible that you really have to have great sounding instruments with great players and a decent sounding space to record them in (either a dead space or a nice sounding room) before you start worrying too much about the technical aspects. No matter how good you are as an engineer, it's really tough to make a bad sounding instrument and a bad player sound good. I used to work with some heavy/death metal guys at the big world class studio whose bass player just didn't have the technique to play good... when he was trying to play fast, he would get this really "thwappy" muddy sound because he was just kind of tapping on the strings instead of pulling or "plucking" each one properly (or whatever the term would be). When he played slower passages with long notes, he did just fine. I tried to explain to them that there wasn't much I could do to give them that crisp defined bass tone with the sounds he was producing... so, the next time they came in, the bass player had gone out and purchased a several thousand dollar Pedula (?) bass, thinking that it would solve his problems. It sounded slightly better, but certainly didn't improve his technique any.
As far as acoustic guitars go, I have had the opportunity to work with many of the top session players here in the Seattle area, and they can definitely make just about any guitar sound good. The guy I worked with a lot would always bring three or four really nice acoustics along with him and let me choose which one I thought sounded best for the particular. He had (I believe) a Guild, Martin, Takamine, and Taylor (can't remember the model numbers). For the pop stuff I was doing, I always liked the sound of his Taylor the best and usually went with that, although I picked the Guild or Martin for some other tunes. Another acoustic singer/songwriter friend of mine had an awesome sounding Takamine as well that really worked great for her. I also always had a negative image of the Ovation guitars, and never liked to record them.... I thought they worked better in live situations and would never consider using them in a studio, until I was working on a big album project with Jill Cohn, and her guitar player had a really nice, higher end Ovation (I didn't know there was such a thing) that actually sounded great in the studio. We did some tracks at the big studio as well as in my project studio, and it really suprised me at how well it sounded in both situations, even on songs where the acoustic guitar was pretty much the only instrument. Of course, he is an excellent guitar player as well, and that helped a lot, but he also knew how to pick a great sounding instrument for him. That was kind of suprising since I had the generalized notion that Ovations simply did not record well (from my limited experience with the cheaper ones I guess).
It is primarily the combination of the player and the instrument that is the biggest factor in sound quality. Even in the hands of pros, there really is no way to generalize about which brand or model guitar or other instrument is going to sound the best, since you could have two guitars of the same make and model that might sound quite a bit different from each other... I think there are too many variables and too many ways to set up an instrument like a guitar. A pro will know how to choose a great instrument to begin with and will be able to set it up properly for his/her playing style.