Since Blizzard wants SC2 to be a competitive game, it will probably run very smoothly, especially if you desactivate most of the spacial effects. (But since my computer has a hard time running even SE5, I think I'll have to buy a new one anyway).
Due to nostalgia I reinstalled SC1 and have started going back through the original campaign. Once I get to Brood Wars I think I'm going to take my crack at battle.net, but I'm certain that years of natural selection have left everyone on b.net in the rush mode that I left it.
I'm sad to hear the Blizzard is tuning SC2 to be a quick pace game such that they predict an "average" game to last 20-30 mins and "Pro" games should last 15.