<< Win 2000, by far... more compatibility, less bloat. >>
As I recall, via Task Manager, XP, off a fresh install/boot takes less memory then a Win2K fresh install/boot.
And with tweaking services, I can get XP down to a small amount of memory.. from 74.4mb on a fresh install/boot to 60.2mb..
I've seen it as low as 54mb with major tweaking.