OC 750TI aint get much... ~2-3fps... but you can OC CPU A LOT... I'm currently running 29% OC, and I prolly could get more, but my mb/ram dont like that many Mhz bus.
fyi. stock q9550 = 8,5 x 333 Mhz
im now stable at 8,5 x 430 Mhz
i could go up to 8,5 x 450 Mhz, but system (in like 99% of times) BSOD on start with memory_management issue, going over 450 system wont start at all.
if anyone wanna more info, Q9550(8,5x430Mhz);4x2GB ADATA Extereme CL4;Asus P5KC(newest bios:1203);cooled by Seidon120V2 even on hot day <66C.