thx albert!

i dont think the boost will be that big... from what ive seen k10 really doesnt scale that well with extra memory bandwidth cause the imc is saturating all the needs.
same with core i7, as soon as the memory controller is integrated into the cpu there is a huge amount of bandwidth and the latency drops notably, so memory bandwidth doesnt scale that well. i actually dont think you need a beefier psu for sli, check the anandtech article about this, the guy who does the psu roundups wrote a nice article on how everybody thinks they need huge watt psus when they actually dont
and the more watts you have and dont use, the less efficient your psu will work and the more power you actually waste.

even a gtx280 is "only" around 200W, a cpu is less than that, and those are the two biggest power consuming parts in a pc...
add another 100W and you cover the rest of the system easily. so for a gtx280 sli system you would need around 600W, and a 750W psu should cover that.
of course, if you plan on ocing or your psu is called 750W but is actually a 600W psu, then you might need a beefier one