It sounds like your problem is that you don't have 800x600 defined in your
XF86Config. I run at 16 bit color depth, so here is what I have:
Subsection "Display"
Depth 16
Modes "1024x768" "800x600" "640x480"
EndSubsection
The first resolution is the default resolution, the following are the
*supported* resolutions. So in your case, you should just add "800x600" after
"1600x1200" (you might want to add the intermediary resolutions in between.
Each color depth has a section like the above, just change the depth value. If
you do this, you won't have to start X in another resolution for the game to
run fullscreen.
To answer your question of starting X in 800x600 without having to alter
XF86Config, I'm not sure. It may be possible, as I know that you can define
the depth via the argument 'startx -- -bpp 16' where '16' is the color depth
you want. I looked at the startx and xinit manpages and I didn't turn up
anything, but maybe someone else will know.
-Rob
