Upgrade vs Full

I currently have XP Pro on my old system and plan on transferring it to my new system as part of a duel boot of that and Vista 64bit. I also have a XP Home that I will put on the old computer that I removed XP Pro from. I've heard of the "trick" for installing an Upgrade Vista onto a PC without having to kill an XP License but honestly, is it worth the risk that MS won't eventually patch that hole leaving people with an Upgrade CD that may be worthless for any future reinstalls? I say worthless because I plan on going 64bit which MS says you can only Upgrade to from a XP 64bit OS. Since I only have 32bit XP, I was thinking that maybe it isn't worth the risk and that I should just bite the bullet and get the full package.

Guess that'll be Ultimate Full (ouch$$$) because I really don't feel like waiting X number of weeks for a 64bit DVD to come by mail while my new system is ready to go. Also I don't think OEM is the way to go due to the limit of only installing it on 1 system. For this kind of money I expect to get some LIFE out of that thing...

Any thoughts on this stuff?

nobody has any thoughts on this? Nobody even has an opinion on whether MS will close the upgrade disk, fresh install "Hole"?

But with Full I can remove it from 1 system once I replace it with a newer rig and install Vista on the new system. With OEM MS only allows you to install it onto 1 system. You can reinstall onto that 1 system as many times as you want, but you can't transfer it to another system.

only with the same model I thought