OS: WM has already made the transition to vista and therefore should run on Windows Seven without problems. I can try this out, when I get home.
You will want 64bit - 32bit makes less and less sense as modern PCs get more and more ram (i.e. >2GB) and the support is pretty good already (imo) and costs just as much as 32bit. XP64 is pretty old and sometimes lacks support because nearly nobody seems to be using it and there is no real need to, considering Vista and Win7, except for maybe(!) a few percents speed-up.
RAM: If you’re on a tight budget, I’d recommend you wait for WM 2.2 which comes with memory management that can page out stuff and thus avoid out of memory errors in some scenarios. If not: Don’t hesitate putting 4GB+ into your machine. RAM is currently ridiculously cheap.
Yes, WM Pro can use multiple cores.
CPU: Don’t know. But you will want a fast one, which will probably mean 4 cores or more.
Graphics card: WM doesn’t really use it except for the 3D, 2D and layout view and you don’t need a terribly fast one for this. I guess every card currently available will be able to hande this. Probably even an onboard chip.
by the way: You should consider using a RAID. I’ve noticed on my 4GB 4 core machine that the hard disk is the one remaining bottleneck.