Why is 1600x1200 not an option? And what if you have 6 computers, most with different settings?

1920x1200 - laptop (15.4" widescreen)
1600x1200 - 2800MP dually (19" CRT)
1280x1024 - main fileserver (17" LCD)
1024x768 - PVR box (15" LCD)
800x600 - old-school Win98 gaming rig (on junky 17" CRT)
800x600 - HTPC (TV-out to projection TV)

So my answer is... whatever the monitor allows for!