Unless you're really unhappy with how your 7950GT runs at whatever resolution/settings you're running, its probably better to wait for a DX10 title you actually want. Yes the 320MB is an upgrade in performance, but driver issues with the 8800s are enough of a concern to marginalize that performance delta. The 6 and 7 series have more or less mature drivers in XP, the 8-series still has a lot of unpatched bugs and issues under XP, and all 3 series have outstanding issues in Vista.
By the time there's a real need for you to upgrade from a 7950 (X1900+ owners are in the same boat), 8800 prices will have dropped and there may be more options like the 8900/8600 series and R600s.