My card is a nvidia card 8600m and for laddering purposes I set everything to low, desert strike runs fine until there are more than 400+ units on screen (and oddly it doesn't overheat on desert strike). My old ati cards ran fine on most games, I think its an sc2 issue, A mobility HD4670 runs sc2 on 5-10 fps is not funny at all.

Are you using an XPS16 model? My bros is using mobility HD4850 and it runs fine(can run high with no problem). Its just that my friends ati HD4670 and HD5450 model lags terribly in fights (even at lowest settings).