I just bought a Dell off their site with Vista as well. E521 - Anthlon 3800+ dual core with 1 GB ram and 250GB sata drive and other goodies. $300 but with no OS option other than Vista. I'm going to get an OEM version and reinstall XP. I plan to get all the drivers off the net for XP before I do anything with the new PC, though. Hopefully no issues, but for the price, I'll deal with the temporary inconvenience.
Why only 1GB of RAM on a new computer? Keep Windows Vista just turn of the “areo” windows interface:
http://www.vistaclues.com/turn-off-the-fancy-windows-vista-aero-interface/
It will look like Windows XP but give you better security.