Careful, check out the GPU charts.
The 8800Ultra is still a plenty powerful card, and in higher resolutions with AA/AF enabled will outpace a 4870 most, not all, but most of the time by a considerable margin. It along with the GTX have the 384 bit wide memory buss, and that makes a difference when you start cranking things way up.
Nvidia did their homework on these cards.
While they are older tech, you cannot pass them off as slower because they are older.
Best advice, check out the GPU charts for the games and res you play at before you make your decision.
You are aware that the 8800 Ultra loses to the HD 4850, let alone the HD 4870... right?