How much money have you go to spend? That's gonna narrow your choice down for a start. As usual with computers, get a board with as much memory as possible. Minimum of 64mb but if you want to play high spec games you're gonna need at 128mb. Basically DDR is faster than SDram.
Personally I have a Radeon 9500pro which I am very happy with but I am sure you'll get a lot of people offering alternative suggestions.