YES! (Certain things depend on your processor and memory, but in general this card is more than capable of running SC2 at medium-high settings).

I'm running Windows 7 with:
Phenom 2.3 Quad Core
4G of DDR2
1G Radeon HD 4650

Windows Experience Index shows a 6.5 rating for gaming graphics with the card.

The settings below run SC2 without any problems even in large battles.
(NOTE: I put it into windowed mode for the Screen Shot, but I normally run  1280x720 Full Screen)