When gaming only one monitor will be displaying the game, so any card that can game at 16x10, which is just about anything $100+ for acceptable eye candy, and the budget cards just under $100 for "ok" performance.
Displaying the Windows desktop and running anything that isn't 3D will not tax *any* card on the market, unless you spend like $10 for a total piece of crap. You do want to make sure to get dual DVI ports, cause lots of cards don't have that. It's an easy /facepalm to make.