Mine ran pretty good a slight jerk here and there panning around zoomed in but other wise it was great. I was also able to Alt/Tab as well without incident. It only bogged bad, but was still technically playable, when the Drengin super gave them 25,000 ships.
That was Huge, 6opp, Graphics at best with 4x AA, 1024x768
Athlon XP3000
512MB PC-2100 DDR
BFG GeForce 6800 O.C.
Virual Mem. at 4GB
You may just need to do some maintenance and tweaking of your setup. I see a few people with good setups like yours, saying they're having slowdown issues. I think a lot has to do with how efficient and stable they have their systems running. I worked a while getting every last bit of performance out of this old system, and it seems to stack up in games to PCs that are quite a bit newer because of that. So if your having bad slowdown with a system with better specs than mine. It's most likely your system.
Edit: I accidently had XP2000 sorry bout that
