If you want XP preloaded (it can be a bitch to downgrade from Vista to XP on a Vista factory system), I highly suggest either getting XP by the deadline date, or, going with Vista Premium - with Vista Premium they should automatically ship XP 'downgrade' CDs, but make sure to ask.
I bought a Vista pre-loaded system, almost smashed it into a million pieces because Vista is such a POS, and finally swapped to XP, which is a night and day improvement.
And I have 4 GB of RAM and a Penryn Processor (Core 2 Duo T8300 2.4GHz 3MB L2 Cache 800 Mhz FSB).