first off, i dont not believe there is a direct upgrade path from XP to 7, so you will have to have the full version of 7. you also need bootcamp 3.0 to run windows 7.
Legally speaking, Microsoft allow you to upgrade from XP to Win 7 using a Win 7 upgrade version. However if you try, the Win7 installer will insist on doing a wipe/fresh install. If you must upgrade without installing a fresh copy, there are a few routes you can go depending on where you're starting and where you want to end up:
XP Home -> Vista Home Prem -> Win 7 Home Prem
XP Pro -> Vista Business -> Win 7 Pro
If you want to upgrade from XP home to Win 7 Pro, the upgrade path would be:
XP Home -> XP Pro -> Vista Business -> Win 7 Pro
Going through all those upgrades, I wouldn't like to guarantee the stability of the resulting operating system though...
Mike