Any card will work. Nvidia or ATI. It has a PCIEx16 slot which is compatable with anything.
Look at this article for suggestions: http://www.tomshardware.com/reviews/fastest-graphics-card-radeon-geforce,3085.html
Also if you don't mind waiting, jan 9th (maybe) will be the release of the HD 7xxx series cards. They'll probally be expensive though.
Your psu and case may cause problems though, what are they?