Don't go nuts on the upgrade for this game, unless you just really want an upgrade. The beta looks beautiful and plays smoothly on my PC which I haven't upgraded in almost three years.
AMD 64X2 5200+ (no OC)
2GB DDR2
8800 GTX 768MB
Not only have I not upgraded, I actually downgraded. I used to have 4GB of ram and dual 8800 GTX's, but I pulled out some ram and one of my 8800's to put in my wife's computer so she could play Sims 3. I thought I wouldn't be able to run SC2 very well, but I had zero problems with the beta.