you could always do what i have and add another 5850. Crossfire 5850's are about equal to a GTX 580/ 7950 and would be enough to max out most of today's games with good fps
Unfortunately, i'd need a new mobo to crossfire, so for cost...I'd lean toward single card.
Sounds like 7870 or 7950? Can the 7870 OC safely? I never could get the 5850 to OC without screen issues, so I abandoned it for stability.