Don't think the Dell configurator will allow you to buy a machine without an OS. But -- when I bought my Dell Optiplex desktop, I requested that they ship it with XP. It comes with a CD-ROM which allows me to upgrade it to Vista Business. As far as I know you can downgrade to XP if you have Ultimate or Business, but not if you have Home or Media.
Never even even thought of "upgrading" to Vista - I have quite enough hassles with another Lenovo Laptop that came with Vista installed ...
And while Win 7 sounds like it will deliver what Vista should have delivered - unless you feel like endless frustration, NEVER EVER purchase or install a Microsoft Product within two years of initial release - been there, done that, got the scars on my back ...