If you're looking to buy 1 mic, and you're not planning on beating it up, I suggest a large-diaphragm condenser. Very versatile for vox, acoustic, amp mic-ing, etc. I found the SM57's lacking to my ear, but they do sound hella good pressed right up against a Marshall speaker.
Musician's Fr. is selling the Oktava Mk219's for $79 each. Awesome deal for a really great mic. Nothing in the price range can beat it (subjective, of course). It does require phantom power.
If your mic requires phantom power, you will need a mic preamp or board with built-in phantom power between the mic and the Audigy. I just got a new computer last week w/ the Audigy Platinum, and I'm already thrilled that I don't have to get on my hands and knees behind the computer desk to plug headphones / lines-in / mics into the card! Great multipurpose card.