Abovementioned Tomshardware link is always a good place to start. Once you've looked that over, and gotten a general idea, start reading a little more in-depth on the cards you pick out.
Right now, the GTX570 is a really good choice, but with AMDs new line just around the corner, followed by nVIDIAs a while after, I'd honestly wait for both to show their new decks.
Obviously the power-per-dollar ratio is the biggest factor in which card you will choose, as it should be, but given two semi-identical choices, I'd personally recommend going nVIDIA.