JunUnplugged,
It seems that the absolute best mics for recording acoustic guitars are condensors, such as the MXL 603s and Oktava MC-012's....and of course the more expensive models. You can purchase the MXL 603s separate for around $70. I go to my favorite store for these...eBay...and buy them new. Of course, you would have to have some sort of a Mic preamp for a condensor, such as might be built into your mixer.
The SM57's are ok for micing acoustics, but are better for micing amps, drums, and sometimes vocals, depending on the singer. I say most of this not so much as from experience, but more from what I've read from posters on this and other sites. I will be getting some SM57's soon, and just orderd an SM58, bundled with some other equipment on eBay. I so far own several Condensors of my own, and 3 cheap CAD 10 dynamic mics, so I need the SM57's to round mine out for basic recording needs...