The new Nvidia GTX 960 would be about the top end card you could use w/o major bottle neck. Nvidia recommends a 400W minimum PSU for it. Another option would be an R9-280. But you'd need at least a 500W good quality PSU for that card.
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100007709%204018%20600537575%20600489223&IsNodeId=1&bop=And&Order=PRICE&PageSize=30