To look at a system setup like this and not also see how the power consumption increases would be crazy. Using two 6800 Ultra cards will certainly increase the pressure on your power supply as well as your power bill.
I tested both the single GPU and dual GPU system at idle (sitting at the Windows desktop 30 seconds or so after boot up) and during a load situation (running the 3DMark05 Game Test 3).
When adding in that second video card, your idle power consumption went up 63 watts, or 39%. During gaming, expect that load to increase by 72 watts or 26% or so. A 348 watt usage on a system that is only running a single SATA hard drive and IDE DVD drive is quite considerable.
Interestingly, while using this SLI setup on my test beds, I got a warning message from my APC battery backup that I have attached to the test computers that I was down to less than 5 minutes of battery usage time when I had the SLI’s going full blast. I guess it’s time to upgrade my 1500VA systemâ€¦