Socket 775 is pretty old now if you can find a cpu it would probably be secondhand.
If you can afford it I think you should do a whole platform update. Cpu, Mobo, Ram.
Core i7 2600k/ ASUS P8Z68-V Z68/ 8gb Corsair Vengeance 1600 Blue/ EVGA Geforce GTX 560ti Maximum Graphics Edition/Windows 7/Antec 300/Antec 650w/LG Blu Combo/Hp 27inch 2710M/Samsung P2770HD
"I think sharing is overrated...and helping others. And what's all this crap I've been hearing about tolerance?"