If you buy Dell, you should get a XP CD. It is very similar to the OEM install. The main differences is that with the Dell CD, you don't have to type in the key (yay!) and it comes with the Dell stamp, otherwise everything should be the same in comparsion to a non-Dell XP CD - or so from what I've observed from doing a clean install with it.
You'd probably have to find out the dell key manually or thru some third party script since they usually don't put the key on paper so yeah, I don't see why it wouldn't work using a non-Dell XP CD as long it's one key or license per computer.