a mic preamp are one of the things in the chain of audio that can make or break a sound.
true, it's job is to boost the signal coming from the mic, but not all mic pres have the same circuitry in them. Just like anything, usually the more money you spend the better quality parts it's going to have. And in audio, the better quality parts usually means better quality of sound. Some preamps color the sound to a perfect amount giving you just that extra "oomph" to your sound. Other preamps might have tubes in them...and as a guitarist will tell you, tubes can add warmth to the sound. And then you have crappy preamps that may cost you $50 for 4 of 'em, but again, just like anything else...you're sacrificing quality for quantity.
No, a preamp will not make just any microphone sound better. There is an audio chain that you have to be careful about. It first starts with the musician...if he's not good, the song will sound like crap. Next is the room, if the room sounds like crap, you'll get crap. Next is the microphone, then the preamp, then the A/D converters, and so on. If the microphone sucks, a great preamp won't do any good. But if the microphone is great, the preamp will make it sound just that much better.
what does you band already have in terms of audio gear? maybe someone here can point out something that you should invest in first before a preamp.