It depends upon the version of directX used. Dx11 is vista/7 only (unless you hack it) so any emulator that uses dx11 can't run on xp.
I know demul does this but afaik it's the only one.
Equilizer:
I don't think you understood my answer. Upgrading and OS does NOTHING for emulators. You are simply upgrading the OS, which is irrelevant because if using a FE your goal will be to hide it as much as possible. Win 7, in terms of just the os and nothing more, will eat up more ram and other resources than xp and it's UAC and similar foolishness, makes it a bad choice for a mame cab.
That being said. Running a 32 bit OS when you have a 64 bit processor slows down the processor. So you should always run a 64 bit os if you have a 64 bit processor. Mame and a couple of the other high-end emulators benefit a lot from running 64 bit versions of their programs.
So 64bit win7, probably not, 64 bit XP... yes, that's what you should probably use.