can't center X display

[Steve Kleene <skdeb@syrano.acb.uc.edu>]

On my Etch machine, I've tested the centering of the desktop under three
conditions: running fvwm, gnome, or a VMware virtual machine in full-screen
mode.  If I set up xorg.conf for 1600x1200, any of the displays is centered
correctly (by luck, I guess).  If I set it up for 1280x1024, each is offset
to the right.  I haven't figured out how to fix this but have tried a few
things.

I called xvidtune, which promptly said that my video wasn't tunable.  Turns
out that my Etch install had left me running the "vesa" video driver.  I
changed xorg.conf to list the "i810" driver, which should be better for the
Intel 82946GZ chipset on my onboard graphics card.  After that, xvidtune gave
its usual display, but if I hit "Apply", even without changing any of the
paremeters, it complained:

  "You have requested a mode-line
   That is not possible, or not supported by your configuration".

I also tried adding a ModeLine to xorg.conf:

Section "Monitor"
        ...
        ModeLine     "1280x1024" 157.50 1280 1344 1504 1728 1024 1025 1028 1072

This didn't cause any errors, even in /var/log/Xorg.0.log.  However, changing
the HSyncStart and HSyncEnd values in the ModeLine had no effect.  (I
restarted X after each change.)

I have not tried installing the i810 driver from Intel.  The instructions at

  http://www.intellinuxgraphics.org/install.html

suggest that I might need some other modules (i810, drm, agpgart) loaded into
into the kernel.  Lsmod showed none of those.  I tried loading them with
insmod.  The first two loaded OK; the last complained that there was no
/dev/agpgart.  Before proceeding any farther I decided to post the question.

Any idea how to get the display centered at 1280x1024, with or without
xvidtune?  Thanks.