They should fit fine, and draw around 300W absolute max. 750-800W is enough, provided there's plenty of 12v. Just don't run Furmark, that will really cause the GPU power usage to spike.
See the SLI / Tri card article posted at AT today for power consumption differences between normal games and Furmark, it's a huge difference. Keep in mind that article is using 1366 CPU and total consumption is likely a bit higher than you'll see with an 1155 system.