If your going for performance in Z have a look on CG talk in the Zbrush thread, theres a thread on gettig the most out of your Pc with Z by tweeking some settings, though other than the 3 gig switch and upping the mem size (in Z) I've not done anymore.

With the 3gig switch I've not have any problems with it on 2 PC's, seems to work a treat.

Rage is right when it comes to the OS and the hardware issues, matter of progression.

I persoanlly wouldnt go to 64 bit unless I was going to have at least 8 gigs of RAM in the system, just to have the overhead and the firepower to take advantage of the platform and the messing about with softwares etc etc, just my opinion of justifying the outlay!

Though as I say I might when I get some cash.