GTX 280 SLI uses about 320 watts. I don't think the rest of his system is going to consume more than ~150w, which would put him in the 450-550 watt range. This is within the comfortable range of the corsair 750w.http://www.anandtech.com/video/showdoc.aspx?i=3539&p=22
Here we have a core i7 965 (130w CPU) and a 4870x2 pulling 421 watts at full load. According to tom's article above, gtx 280 sli uses about 50 more watts than a 4870x2 while gtx 260 sli uses 100 more. From this review
it looks like the gtx 275 uses about as much as a gtx 280. If we add 50 watts to the 421 watts from the anand system, we'd get ~470. The OP's CPU might use a little more than the stock i7 (a Q6600 is 95w at stock), and a 2nd hard drive won't be a whole lot more either. Other than that, the systems are pretty similar.
I still think the OP's current PSU will be sufficient. Won't be able to add much more to the system though...