I tried hard to make 3D acceleration work for my Powercolor HD 3850 AGP on the following system:

Intel 865 mainboard (Gigabyte GA-8I848P-G)
P4 2,8 GHz 512MB RAM

I used fglrx drivers from Catalyst versions:
8.7
8.8
8.10
8.11
8.12
9.1betaRC2

- on Ubuntu 8.10 kernel 2.6.27-9-generic
- on Debian/Lenny kernel 2.6.26-1
by building debs from the installer, plus one time direct install.
(cleanup between installs by dpkg -r).

everything installs OK, AGP initializes to 8x (intel_agp), DRI initializes, no real problems in Xorg.log visible except several warnings like
...
(WW) AIGLX: 3D driver claims to not support visual 0x72
...

now the problem is: every time I start an OpenGL app (glxglears or fgl_glxgears will do), the system freezes:
With earlier drivers from Catalyst 8.8, fgl_glxgears runs for about 10 seconds; the more recent driver I use, the faster the freeze takes place:
With 8.8 I can run
"fgl_glxgears -loop 500"
several times successfully, until system finally freezes in the 10th or so run.
With 9.1betaRC2 the cube just slightly starts turning and the system immediately locks up.
This somehow makes me unsure when hoping for upcoming driver releases to resolve the issue....

I've been trying a hell lot of xorg Options and Option combinations from
http://debian.wgdd.de/temp/fglrx_man/fglrx.4x.html
without any noticeable change.

With 8.12 I have pretty good speed in normal 2D apps, fast scrolling with Firefox, moving it around with several tabs open is no prob, too. System runs without trouble until any OpenGL app is started. (OK, I didn't test user switching and suspend - I had enough with the normal stuff...).

On this same system, Catalyst 8.12 Windows XP drivers work fine, full AGP 8x speed, 3DMark06 runs to end without any errors, so I guess there's not a heat or PSU problem (510 W, by the way).

Anybody a good idea what to try next - beside the obvious "go for nvidia"?

TIA
maddoc.