Windows 7 Pro, 64 bit, 4GB
Interestingly that's the exact specification on the 4 desktops and one laptop in Dastard Mansions!
One thing, though - if you have any 16-bit software it will definitely not run at all on 64-bit, while it might run on 32-bit versions of Win 7.
I've never tried running 16-bit apps on 32-bit Win 7, but I'm assuming that if it ran on 32-bit XP it might still work on 32-bit Win 7.
SD