dont know the brand of your ram but you should be able to clock the timings to 4-4-4-12 on 2t pretty easy. if its gskill you could o/c it to 1000mhz and still push the 4-4-4-12 timings.
the answer to your questions is no, the psu's will only use what power is need the 750 will not use more than the 650. i would get the 650tx that psu can handle an i7 and a your gpu easy.
i think newegg.com have something like a build setup where you go through and chose the parts for your computer. but this is what i recommend.
cpu: i7 920 $280
cpu cooler: core contact $50 with i7 bracket
mobo: biostar tpower x58 $250
gpu: gtx...
that gigabyte mobo is the way to go, p45 chitset is the best overclocker for the 775. if you can get a biostar p45 i would get that. biostar boards are know for holding overclock records.