I would recommend 700W and up for running dual cards of that caliber.
I just ran newegg's power supply calculator and it came up with a minimum 669W. Thats with a dual core CPU, 2x8800GTS (they didn't have 8800GT's in the list so this is probably the closest thing in terms of power consumption) 4Gb ram, 2 CD/DVD drives, and 2 internal hard drives.
you may want to wait until someone with a bit more experience with SLI replies.
http://img91.imageshack.us/img91/71/88062081ls8.jpg