Don't be too worried about the GUI. Mine doesn't show anything higher than 640x480, but I have TWO 1920x1200's working at native resolution.
Says in your xorg.conf in the "Screen" section that Modes "800x600". It pretty much means what it looks like. Change it to what you think it should be.
Also note this; intel cards are known for having very limited modes available in the video bios. 1600x1200 may not be in the list, in which case you may need to use a hack "915resolution".