You Need Phantom Power
I agree with the others on this thread that the best pure acoustic sound will be with a combination of condenser mics. There are many ways to do this and the best way for you will come from experimentation. You should be able to get a really nice and clean, crystalline sound with a good guitar, fresh strings, and a decent guitarist.
However, your recorder does not provide phantom power. So if you are going to move from the dynamic / SM57 class of mics into the condenser class, you will need to address that. I think the best way is to do it with a decent inexpensive outboard preamp, as that will provide both phantom power to run the mics and will also be a noticeable improvement over the Fostex preamps. Not to bang on the Fostex, but it's an entry level unit and preamps are where compromises usually get made.
Here's my suggestion. Look around for a decent two channel preamp; I'm always plugging the M-Audio DMP3 and will do it again. Mine is quiet and reliable, and for about $150 - 160 it's an excellent value. There are other good entry level preamps available as well, but this particular one is a no-brainer to me.
Then look for a decent entry level condenser mic package; stay with recognized brands because in this price point you can encounter a lot of schlock. MXL, CAD, Studio Projects, all will have decent mics for not much money that will do the trick and help you learn.
I would look for a combination of a large diaphragm condenser and a small diaphragm condenser. My favorite recording position is to point the small unit at the sweet spot, around the twelfth fret and a foot or so away, a little above the guitarist, so it's pointing at a five degree angle down. I then set up a side address large diaphragm mic about three feet away and pointing to the bridge; I try and stay away from the sound hole.
My limited gear (that I love) is a Studio Projects B1 large diaphragm condenser mic and an MXL 603s small condenser, both using shock mounts on a stand or boom. Figure on spending around $160 for the preamp and $ 150 - 200 total for the mics, and toss in a few bucks for cables and patch cables from the preamp to the Fostex - you'll be grinning in no time.
Be careful of condenser mics, as they are fragile and the 603s can be damaged by high sound levels or vocal plosions ("p," or "b" sounds accompanied by a burst of air) - not great for vocals that way.