That's quite a big question!
Generally speaking, the most important thing is to put the performers in a situation where they perform to their best. A very high quality recording of an awkward performance is much worse than a slightly dodgy recording of a spellbinding performance. So within reason that's priority 1.
I find acoustic pickups sound much worse than a miked up acoustic, so I'd always try to mic the instrument first. Generally having a cardioid mic about 30 cm in front of the 12th fret, pointing at where the neck joins the body, is a very good starting point. If that's too bright/thin, you can turn the mic further towards the body or move it in that direction. You can also move it up or down and you'll find that the tone changes. While the above is a good place to start, it's worth experimenting. There's some gorgeous midrange that comes off the body of an acoustic if you put the mic in other places - pointing over the player's shoulder, pointing upwards from underneath the guitar etc. Generally it's good to avoid putting the mic in line with the sound hole because even at a distance the sound coming from there can be quite woofy and boomy.
As for the mic used, a '57 or '58 works for acoustic if you're happy with a relatively mellow, earthy sound. You'll miss out a lot of the clarity and liveliness that a good acoustic has, which may or may not be a problem for you. Also you'll want a quiet mic pre, as unless it's an unusually loud acoustic you'll have to crank the preamp gain quite high to get a decent signal recorded from 30cm away. My temptation there would be to move the mic closer, but then you start to focus in on a smaller part of the guitar. Alternatively there are quite a few low cost condensor mics, I've only had experience with some, but the AT2020 is one I tried that's cheap and surprisingly good, also the Sontronics STC-1.
If you're wanting to capture the sound of someone singing and playing guitar at the same time, then mic positioning becomes very important. Ideally you'd have two mics with figure of 8 pickup patterns and set them up in a kind of "X" shape so that the vocal mic had the acoustic at right angles to it and the acoustic mic likewise was side on to the singer's mouth. The really limits where you can actually place the mics but if spill is a problem it's your best bet. Personally I'd record the acoustic and vocals seperately unless it ruins the performance!
If I had to record someone singing and playing acoustic and only had a '57 and '58, I'd set up the '58 pointing upwards as much as possible so the guy was singing downwards at it, and I'd put the '57 as close to where the neck meets the body as I could get it without it sounding too boomy (the closer you get the more bass the mic will pick up, thanks to the proximity effect) pointing as far from the guy's head as I could get it while still pointing at the guitar.
If any of that is poorly explained or confusing let me know!
