Depends. If you have a lot of old software, then you should probably stay with XP. It is a decent OS.
Win 7 Pro and Ultimate will ship with "XP Mode" (actually 2 CD's-one is a modified Virtual PC and the other is an XP "image in .iso form.
Then again, your processor would have to support Virtualization, and Virtualization does have a performance penalty attached to it.
So, stick to XP, or upgrade to
UBUNTU , it's FREE! LOL