Hi newcomer and welcome to the Tom's hardware forum.
Like I see ur rig is for gaming, so, usually we uses Win 7 Professional or Ultimate both Professional or Ultimate x64.
Now, u can keep the XP license in the old rig and the new one in the rig that u are building since is impossible do a update from Win XP 32 bits or Vista 32 bits to Win 7 x64.
Buy ur update version and with the update DVD u can do a clean installation of Win 7.
Ahh - I'm not even sure what benefits the other versions have - I'm not too interested in anything other than the basic Windows 7.
Thanks for the welcome.
I'm loving Toms Hardware - lots of great info and help when you need it!
So how does it work then - when I go to install Windows 7 on my new computer, I'm guessing I have to give a cd-key type thing from my Windows XP on my old computer, then it let's me install the upgrade version as if it was the normal full version?