Seeing as how it's Blizzard, I'm betting it'll scale very well... so basically if you have the hardware then it'll look fantastic but if you don't, it'll still run so you can at least play it.
I'm guessing the minimum will be at least a Athlon or P4, 64MB dedicated DX9c video card, 512MB of ram and a lot of HD space for an optional HD install (probably 4GB).