FYI: According to AMD you can put SLI 6870's on a 600W PSU (I'm probably going to end up trying it myself 2-3 years from now)
The overclocked i5 will draw ~120W max load. The GPU will draw ~150W max load. Throw in everything else and you are around 300W max load.
EDIT: The 8800gt draws 100W max load, so SLI would be 200W. The older 6000+ had a 125W power draw. It should have ran on a 550W PSU, but it could have been bad, a cheap brand, aged poorly, or any number of factors.
Incorrect, it's more like 112w.