your processor is too weak. SC2 is a processor hog. if you can afford a c2q you'll be much better off. one of my friends drops down to 22ish fps and he has a gtx470 paired with 8gb of memory and a 2500k overclocked 4ghz.
Incorrect. A core 2 E6600 handles SC2 just fine, even on large 8 player maps. My previous rig had almost the same specs (the GPU in that rig was actually weaker - 8800 GT). While SC2's system demands lean more toward the CPU than most games, those demands are actually pretty low, as is Blizzard's M.O.
just to reinforce the above posts, your symptoms do sound like a heat issue.