well, you can use either CD I believe, as the upgrade CD is made for this purpose. But if I were you, I'd try to get the normal cd not the upgrade, because if you ever want to do a clean install of XP, it makes it a lot easier to have the real version. After saying that, the upgrade CD is $100 less than the full version.
One thing you should know about doing the upgrade. I have heard (I did a clean install) that your system will be slower with the upgrade than it would be w/ a clean install. My suggestion is if you don't have a cd burner, get one and buy yourself 50 or 100 cd-r and burn everything on to cd and do a clean install. This also makes it easier to reinstall everything if you ever commit a sin XP doesn't like and you have to reinstall. Good luck!