Also, why XP 64? You have the beef to handle Vista 64 and considering it is supported by even the little guys now as opposed to XP 64.
On my machine at work we priced out a dual xeon and a new i7. The dual xeons were a pain in the ass and much slower than a single quad i7 which acts as 8 cores anyways.
Still, it is awesome to have a new rig to put to the paces.