Perhaps overclock the e8400, as SC2 likes 2 cores and higher clock speeds.
What resolution are you playing on, a gtx 460 should be enough to max out at any resolution according to the reviews I have read, of course, most of them were done with an overclocked i7 930.
I have a 460gtx 1gb. I can play SC2 on ultra with about 40 to 60 FPS. I think I get a little more fps in the movies. Fluctuating for 40 to 90 or 100. Avg. about 60.
I have a intel 2.4ghz dual core processor. 2 gigs ram. Playing in 1900x1080 on a 42 inch LCD tv.
Haven't played online yet. Still on single player.
The game doesn't require high end graphics imo. There doesn't even seem to be a huge difference from high to ultra. But I'm not one of those guys that's a graphics whore and can notice all the little details.