Gotta love SM57s, good on just about anything. We even occasionally track vocals with one when someone's got a load of natural grit and rasp. If you can find a decent used one lying around on PeeBay or just happen to have a bit more cash, the Sennheiser 421 would be the logical next step up, with a greater bass response when needed to capture a little more meat off of the amp, and absolutely fantastic on snares 'n' toms.
The reality of digital recording is that you can expect to lose gobs of highs and lows after something survives the trip into the computer. It's normal, but it's not hopeless. The preamp stage can really make or break a signal prior to digital conversion, so as soon as you get the cash, think about getting a cheap tube/valve pre, like one of A.R.T. tube boxes or a DBX Minipre, and run everything through it. The preamps on most cheap A-D convertors will put your signal in an fricken icebox, and as nice as having two channels can be when recording a stereo signal, the preamp you've got your eyes on will probably just give you a nice louder version of whatever you've got already. Tooob preamps will give you the undefinable oomph, and if you're lucky just might be enough to make a difference. I'd save up, buy the right stuff and track later if I were you.
Here are the ART items:
Getting one with a compression stage can be a real help for number of reasons, although Butch Vig would stroke his fuzzy goatee and say otherwise(granted, over compressing does destroy the life out of a track). Still, I run just about everything with even just a little comp to either smooth out a pop vocal(medium attack/release, 3:1 ratio) and such, or colorize the tone like on a thumpin dance kick or pour it on and do an REM/The Church/The Byrds-style jingle jangle on my Jazzy or 12string.
Just me two cents, best wishes.