YEs, a 750W PSU should be ok, provided that it has a powerful 12V rails (preferably single) Not sure if the Thermaltake is up to the task, but PSU's like the Silverstone OP750/DA750 and The PC Power & Cooler Silencer 750W will be.
I was planning on 8800 ultras. and if you say i should not get the 1000w
i would follow your plan.
which one has 4x 6 pin pci-e? and which would would you suggest around the 750/850 mark? i just need a better psu for quad 6 pins, as i don't wanna use converters. and i would LOVE to have it quiet as my thermaltake toughpower is near dear quiet.