My situation is similar to yours. I'm considering an 8800GTS for an upgrade. To justify a full 8800GTX, I'd need to upgrade to a very high end cpu for that socket, which I've also been pondering... but then there's core 2 duo as well.
If I were just swapping up the vid card (and that may be what happens), it's probably gonna be an 8800GTS, unless then new ati R600 based cards offer something better.
Time marches on