Short version of two questions: (1) Why does 'amd64-agp.ko' seem to only greatly speed up 'radeon' X driver (not 'nv' and 'via'), (2) why does using 'gdm' as opposed to 'startx' cancel this effect?

(Note: kernel 2.6.20.6, latest Debian unstable as of 20060409, i.e. xserver-xorg_7.1.0-18_all.deb, gdm_2.16.4-1.1_i386.deb, frame rates are from game njam_1.25-3_i386.deb with 1280x1024-24; no proprietary drivers are used).

Background: Slow video response on AMD64 based machines compared to same video card in slower Intel-P4 machine (example: AMD64 30 FPS, P4 74 FPS )

On the intel machines, I was using "modprobe intel-agp" before get X to initialize. So I did:

ls /lib/modules/2.6.20.6-k8/kernel/drivers/char/agp/

agpgart.ko ati-agp.ko nvidia-agp.ko via-agp.ko
amd64-agp.ko intel-agp.ko sis-agp.ko

I try "modprobe amd64-agp" before allow "modprobe agpgart" on AMD64 machine. I read about amd64-agp and it seems to be the 'host bridge/southbridge driver' and on-chip GART GLX support.

Now frame rate on two AGP cards using 'radeon' driver is increased from 30 FPS to 143 FPS, it is like having much faster machine. But there is no effect on AGP video card using 'nv' driver, still about 30 FPS. I want to make 'nv' become very fast also!

On 'radeon' success, I only get the high frame rate if doing 'startx'. If I allow Gnome login manager 'gdm' instead of manual 'startx', the frame rate is not increased. How can I get 'gdm' to be very fast like 'startx'?