OEM *is* a full copy, just not one you can get on store shelves like in Best Buy/CompUSA/etc. It's the version of XP Pro sold to companies like Dell and Gateway and HP to install on their own line of products. Same software, just typically in some form of brand-specific packaging.
You save money buying an OEM version because typically it will install on any machine - except the Dell ones. They're tied to the BIOS in a Dell machine and usually will not install on anything but a Dell, but I've read cases and heard from people that said it'll install on anything. I don't have anything but Dell hardware to test so I can't say yes or no on that one.
I think you're safe getting an OEM copy as long as it comes with the hologram CD and the Certificate of Authenticity sticker (the COA light green sticker with the Product Key you normally find on the PC it's installed on).
Save yourself a few bucks, find a real OEM copy from someplace and buy it.
Hope this helps...
bb