My setup is this:

I have two 19" LCDs (not too expensive to buy two these days. Can be cheaper than one Dell widescreen for the same or more screen area). Both are the same and have dual VGA/DVI inputs.

The connection is like this:

PC ----DVI----> Monitor 1
PC ----DVI----> Monitor 2
Xbox ----VGA----> Monitor 2

When I'm using my PC and my Xbox is off, I have a dual screen setup. Dual screen is wonderful when you are using a PC - makes life SO much easier.

When I'm playing the Xbox, I turn off the dual-screen on the PC (and use only one screen), and the Xbox uses the screen that I have just turned off. Works very well!