GTX 280 SLI problem

I just got myself a second XFX GTX 280 XT (slightly overclocked version of the 280), and enabled SLI on my rig. When I fired up Fallout 3, I got a few seconds of uninterrupted play, after which the screen blacked out, and the entire system locked up. Tried Far Cry 2 using the SLI setup, and it was even worse: the entire system would lock up before I even get to see the first polygon! Then I tried out Half-Life 2 on the same setup, and I got no lock ups.

I have yet to test the newer 280 on the more demanding games without SLI, so as to determine whether this is a problem with the new card. I was wondering however: I've got an 800W PSU hooked up to my system. Is this enough to power such a setup? Perhaps the problems I am having are PSU related.

Here are the system specs:

OCZ Elite Xtreme 800W PSU
2 x XFX GTX 280 XT in SLI
Intel Core i7 940
MSI Eclipse SLI Mobo
Creative Audigy Soundcard
3 x 1GB Corsair Dominator 1600Mhz DIMM
300Gb Western Digital Velociraptor
Zalman CNPS 9700 NT CPU cooler

Any help would be appreciated!