Thanks all. This has been very helpful.
I guess I'm looking at ~300W and actually SLi can bring down each card's overall peak power consumption which is NOT something I took into account. I guess the big nasty beast was peak consumption, which would be my concern, although I'm looking at a 500-600w combined circuit.
So even with a the 80% power efficiency of my PSU at high temps I think I'm in that grey area of ok. My system runs cool generally with my fan setup, so should that be the case-- I would have an additional 200W unused, having worked it out.
Thanks again Prox and Friedman.