One word of warning: if you buy an upgrade copy of Windows 7, and then try to install it to a new or wiped hard drive, it will refuse to activate. It needs to see an XP (or any qualifying earlier operating system) on the hard drive when it starts to install.
Earlier Windows upgrade install routines would accept being shown the previous install disk. Windows 7 won't. I know this because I found out the hard way when helping a friend to upgrade.