An i7 920 at 3.8 GHz will NOT Bottleneck any card out there today, so don't worry about that at all. Theres not really any improvement in gaming with a CPU thats faster than that one!
Your 2xgtx260's ought to be fine with that PSU- its a good quality one and will have no problem running those GPUs.
The 750 should be NO problem at all. with 60A on the +12V rail, and even the manufacturer recommending a 700W 50A +12V rail, shouldn't be an issue. Most PSU calculators have a fairly large cushion built in, even the manufacturer recommendations generally are fairly cushioned.