Well, at 1024x768, it was smooth, so not laggy, but it wasn't running fast either. Around 18-25fps usually.
The bosses were alright, and yeah, I kept getting forzen a lot, but they weren't really that hard to beat.
Quads are worth getting I think, but I'm not sure how much of a difference in this game, one will do at the moment, compared with a dual. Explosions etc never slowed my system down though, however many were going on at once.
Dual 8800GT's should make the game playable at the high resolutions at full settings I'd say, if mine can do it now
Maybe you should wait for the next stream of GPU's to come out though in 2008, for prices to drop, etc.