1. No one can be sure because (a) your ambient temp is high, (b) dunno what voltage you will need for that 40% OC, and no idea how much heat it would throw if we did.
Best I can say is check any review OCing the i920 (google "i920 review"), check what temp that cooler held the i920 at, and add 5-10C to that to adjust for your ambient (unless they specify the review's ambient, then subtract from yours and add that to the max temp.).
2) Yes, I think the 750W is enough for those 2x275. Corsair (as you probably know) wants you to use their 1000W if you have OC'd an i7 with 2x275 lol. So, i'd probably conced and get the 850W.
Thank you! I will get the 850 then, I build computers for people all of the time, but for my own I want to be extra careful (plus it's like 1 million times stronger than what I build at work, =p). Never be too careful.
Also, concerning the 920 OC it should OC nicely to 3.8 on air cooling with minimal voltage increase, at around 70-80 max temp on my ambient temp. Or so I'm told, it can still hold higher than that so I should be OK.