GPUs performance depends upon, among other things, screen resolution. However, a good card that should meet your needs is a 6870, or a gtx 560.
the 6870 typically goes for less than the gtx 560. both those cards can be had for under 200.00. You can find them for considerably less. After mail in rebates you can get them for about 150ish.
Its better if we know how much you want to spend and the current specs of your pc. Or if you are bulidng a new system fill this out http://www.tomshardware.com/forum/261222-31-build-advice