1. 2.0 is backwards compatible with 1.0, although 2.1 may cause some issues
2. for basically similar performance as your 8800GTS, look into the ATI 4850, 5750 and Nvidia GTS250.
3. The Nvidia GTX260 and HD 5770 are solid upgrades from your 8800GTS and yes your CPU will not push them as much as a faster CPU but you seem to keep your cards for a while and you can always carry over one of these cards to your future PC.
FYI, the GTX260 tends to be overpriced compared to the ATI 5770.