In actuality SC2 has more problems with CPU than GPU. The 460 is fine for the resolution, but the more units fighting the more math the CPU has to do and the more it will bog down trying to compute all the combat.
Lucky for you, you have the AMD Phenom IIx4 955. Meaning if you do experience a bogdown with lots of fighting clock it up to 3.5-3.6 GHz and that should help some.
I have a q9550 @ 3.5 and it handles SC2 on Ultra at 50 to 70fps with a GTX 460 1gb.
do you have an aftermarket cooler? If not, you should buy one for your cpu to take care of that. I'm using a freezer pro 7 and my temps don't go higher than 58c, but before that they were in the upper 70's