well, i can't imagine it would be worse. But no I don't imagine it would make it better. Compared to more RAM or a new video card or other suchlike things anywa.
here's my take. xp is aging, and therefore less secure, and more prone to failure. win7 is actually efficient and stable (i say this as a mac loon). so unless you have some particular reason for using xp (eg. compatibility with older software you have), I'd go the win7 route
re SC2 in particular, the game is not especially demanding on hardware, and win7 is as I say pretty efficient - so unless your hardware is on the aging side, I'd think you can't loose anything by installing under win 7. But it's not going to magic your hardware up a notch or anything.