7750 is basically a dual core agena. as mentioned b4, what u need is a gud board to push it higher. it could be d chipset/board that's hinderin ya oc mileage. some 780 boards are known to have high nb temp issue, double check yas... 
bro, i dun really think 2 * 2GB modules will be d reason why d chip can't clock well, it's diff when it's 4 * 1GB. 
try to hit d max clock 1st by doin multi oc. when u get d max stable clock, then only play w/ htt + mutli (but b4 that u must know d max speed ya ram can handle). oc usually is limited with on board vga is in used, try to oc w/ descret gfx if u have one.

d chipset temp is kinda high to me

Got any restart? I haven't try that high yet.