Nvidia and ATI cards use the same interfaces PCI-E there are various versions of PCI-E though so you may want to check that in the MOBO manual.
The 8800GTX was a good card a while ago, but they were power hungry so you should have a large enough PSU to cope with any card.
You have a HUGE selection of cards available from the Nvidia side you could go the 260GTX, 275,285 or if you have lots of money 295
ATI wise you can go for the 4870, 5850, 5870 but it does all depend on budget and what you use the PC for, as you have an 8800GTX I will assume that you either play games or do video editing (as thats mainly what the higher end cards are for)
You need to have a price bracket in mind and then we can go from there