Luckily the GTS is a bit forgiving, it has been found to be approximately 115w / 9.6a for the single card.
A quick run through the PSU calc estimates your system 12v demand at ~9.5a less the GPU(s), so a basic 8800 GTS SLI system (1 hdd, 1 dvd, 3 fans etc) probably won't pull more than +12v@29A at stock and maybe a comfortable +12v max would be around 37A so you don't break the 80% mark.
So yes a 700w will do the job nicely with room to spare, even a 600w like the ocz gamextreme 600w would have plenty of juice, and not such a bad deal on newegg at the moment, if you don't mind rebates.