Still not clear why you can't go for W7; you don't need to get Vista and then W7, because you can buy a W7 Upgrade package. The difference with going to W7 from XP as opposed to from Vista is that you have to do a clean install; i.e. starting from scratch like a new machine. But as you're getting new hardware anyway........
To find out more detail, you can search for w7 upgrade from xp,
cheers,
Jon.