resolutions & virtual desktop
Easiest way I've found is to set the Mode line with the one you want as the default as the first in the list. Like:
Modes "800x600" "1280x1024" "1024x768" "640x480"
ViewPort 0 0
Starts as 800x600 and virtual desktop of 800x600. You can then use the ctl-alt-+ to switch to the others. Set it the same in the other Depth sections too ;-)
Give it a go and see if that doesn't work.