glxgears slow under 2.6, fast under 2.4

[Ric Otte <ric@otte.ucsc.edu>]

Things are very slow for me under 2.6 kernels.  Under 2.4.16 I was able to
play tuxracer just fine, but under 2.6.6 things are so slow that it is
unplayable (a second or so pause between command and action).  I ran glxgears
under 2.4.16 and got output like:
    2430 frames in 5.0 seconds = 486.000 FPS
Under 2.6.6 I get output like:
    1332 frames in 5.0 seconds = 266.400 FPS
I ran glxinfo and noticed that under 2.4.16 direct rendering was YES but it
was NO under 2.6.6.  I then tried upgrading to 2.6.8.1 (changing the agpgart
chipset to via from intel in the kernel (i do have a via chipset) and things
got worse under glxgears with the correct chipset in the kernel:
    860 frames in 5.0 seconds = 172.000 FPS
I do notice that under 2.4.16 I get the following in dmesg:
    dmesg |grep agpgart
     Linux agpgart interface v0.99 (c) Jeff Hartmann
     agpgart: Maximum main memory to use for agp memory: 439M
     agpgart: Detected Via Apollo Pro KT266 chipset
     agpgart: AGP aperture is 32M @ 0xfc000000
and under 2.6.6 I get:
    Linux agpgart interface v0.100 (c) Dave Jones
    [drm:drm_init] *ERROR* Cannot initialize the agpgart module.
while under 2.6.8.1 I get:
    $ dmesg |grep agpgart
     Linux agpgart interface v0.100 (c) Dave Jones
     [drm:mga_probe] *ERROR* Cannot initialize the agpgart module.
     agpgart: Detected VIA KT266/KY266x/KT333 chipset
     agpgart: Maximum main memory to use for agp memory: 439M
     agpgart: AGP aperture is 32M @ 0xfc000000
So things seemed to improve under 2.6.8.1 in that at least I'm detecting the
via chipset, but I'm still not initializing the agpgart module.  I'm using a
Matrox g550 video card.

Any suggestions as to what might be wrong or what to do would be appreciated.
I'm not a debian expert, and I suspect I may be doing something
obviously wrong.
Thanks,
Ric