What 22" monitor are we talking about?
Every 22" screen I've ever seen has a resolution of 1680x1050.
(EDIT: Ignore the top two lines, I didn't read the first post!)
1080P doesn't fit into that last time I checked, and if you select 1080P on that monitor, you'll loose veritcal sync, and just get a power save mode.
PC monitors don't scale like LCD TV's do.
If you use the VGA cable, you can pick VGA resolutions.
Read this article on IGN for more help.
IGN: Feature: Xbox 360 On PC Monitor Action