I have a 7900GTX & I still see entire barns popping up now & then with my detail set to around 3/4 up, I think you're still in a good range at 75fps if your detail settings are more than halfway up with high textures.
The big thing is, upgrading right now, you are on the cusp of direct-X10, better to stick it out until this winter or next summer if you can & see what's available then. If Vista is ever released the 64bit architecture will get a avalanche of support also, & that is pretty much all the GPU manufacturers are waiting on to release their new DX-10 compliant cards...Vista.