personally Id say more CPU limited, the game seems to be a CPU hog and also a ram hog, Ive actually once seen the memory utilization go over 3.0GB and Ive never seen that in any other game I own so I definitely think upgrading from 2 to 4 gigs (like already said) will benefit you, and I agree with Herr_Koos in turning the AA off, it does kill performance on lower/mid systems.
But honestly (when considering that game only), If you were to upgrade... the first thing would probably be the Ram, CPU, and then GPU in that order... having the Quad should at least bring your minimums up a good bit and moreover it will make your system overall a bit snappier.
I also run SC2 on my rig in my sig below and I actually installed the game on my SSD so loading and the overall experience of the game is excellent, so maybe drop in a 955, give it an oc and see if at that point you need more of a graphics subsystem...