There are a few cards that you can use and it would depend on your budget and your preference for Nvidia or AMD.
From AMD you would have the new release cards 7770 , 7850 and 7870 and then the older cards which are now a little cheaper , 6850 , 6870 and 6950.
From Nvidia you have the GTX 550Ti , GTX560 , GTX560Ti and the soon to be released mainstream cards.
It will depend on your budget and also not going over the 500w psu that you have.
Before buying a video card you do want to look at the spec's of your power supply to make sure it will handle it.
Both the 560 and 6870 are good cards and should work fine for you provided your psu is good enough.
If you were to get the 7850 then that would be the best card of the group and be all the card that you need.