Probably. An 8800GT can draw upto 250 watts under peak load. Two of them working seperately, and I guess 400 wouldn't be unreasonable, combined with a good motherboard and a couple of Hard Drives, you could easily get to 750 watts.

The key with the 8800GT is to have a good +12V Rail. Ideally you need around 24 Amps per +12V rail, then power each Graphics card from a seperate rail.

The only way to be sure is to buy a massively powerful PSU :-D

Personally I find this to be extremely useful when deciding on a Power supply.