It would help us a lot of if you said how much you have available to upgrade your computer.
Iv'e checked your Mboard on the net and you got the x16 slot but your max speed for RAM is 800mhz, so you cant get higher.
Don't buy quad core yet , many games isnt compatible yet and you might change your computer maybe later so keep your cash and buy a i7 920 or (if heard about i5 and i3)
If the price isnt too high for you buy a i5 or i3 (no confirmation yet about those series)
i7 920 is a quad core.