Not enough performance difference to be worth upgrading. I would just stick with the 8800GTX until the new 9000 series from Nvidia and 4000 from ATI comes out.
I'd say no... You may get some improvement - depending on the game - but the outlay of cash isn't worth it. I'd wait for the next generation of GPUs and see what they'll do for you.
Spending that money on a better CPU or more RAM could benefit you more.