SC2 will need to be very scalable to reach the a huge audience like its predecessor did. The game will probably run on most single-core cpu setups (P4 and up) and I'd imagine maybe a GeForce FX series card or ATI 9xxx card will be the absolute bare minimum to get lowest possible settings with a playable framerate.
As for being able to decently max the game out, probably a first-gen dual core processor (S939 X2s or Pentium Ds) and an X1650PRO/7600GT or an X1950PRO or something.. that's what I'm guessing
From what I've seen, this game doesn't look too graphics-intensive at all. I think visually it might be equal to or less than Company of Heroes. World in Conflict is probably the most graphics-intensive RTS I've ever seen.
Don't take my word for it, have a look yourself: http://www.starcraft-2.info/img/screens/screen1.jpg