I am running my X11 in 1024x768x16 at the mo on my thin-client, which
doesnt have Xconfigurator.

I want it to run in 800x600x8. heres some of my XF86Config:

Section "Screen"
Driver "svga"
Device "mediagx"
Monitor "Generic Monitor"

Subsection "Display"

Depth 8
Modes "800x600-75"
Virtual 800 600
Viewport 0 0

EndSubsection

EndSection

in the verbose stuff that i get when trying it in said resolution there is a
line which says:

SVGA: Display pitch set to 1024

my Mode lines for this mode are:

Modeline "800x600-75" 49.5 800 816 896 1056 600 601 604 625 +HSync +VSync

Another thing i tried was to plug the monitor into my main box and run Xconfigurator and make it do 800x600 that way, which it did. I copied the resulting lines from the XF86Config that it produced to my thin-client config. but this didnt work either.

what XFree86 are you running ? cause version 4.....
uses XF86Config-4 by default. Maybe you are
hacking the wrong script ?
cause the syntax in my XF86Config-4 is slightly
different i guess you use XFree86 version 3.3... ..
but i could be wrong as i haven't used version 3 in
ages

Is the "Modeline '800x600-75'"... a valid entry ? Mine doest show the "-75" or any other value only "Modeline '800x600'"

in the said file the Modeline name in quotes i.e. "800x600-75", is an alias.