Hi. This isn't really a post for help. I just want to talk 'bout a problem I had and solved, and didn't find anything that could help on the internet, but many people who had the same problem. So this may help someone someday.
I've got an ASUS A7V8X-X with the VIA KT400 chipset for North Bridge. The thing is, when I installed my MSI NVidia GeForce 4 MX 440 (AGP 8x), and installed the NVidia driver for Linux, when I tryed to start X, I got a black screen, both on the monitor and on TV, and when I tryed to comeback, with "alt-F#", X would quit and say that it had quit by user request. Besides, my console cursor showed only on TV, and I had to reboot to have the system normal again. My /var/log/XFree86.0.log would point no errors, neither my /var/log/messages. But the last thing /var/log/messages pointed was to agpgart initialization.
The system started working when I changed the NvAGP option (in the driver section of XF86Config) from 3 (default) to 2 (or 0, but 2 is better). 3 means that the driver tries to initialize agpgard, and, if it doesen't work, nvidia agp, witch is not compatible with my MB. 0 mens no AGP (real slow) and 2 means with agpgart.
The fun thing is that I can't initialize agpgart by hand (insmod), it doesn't find my chipset, and when X is running it doesn't appear as loaded in lsmod, and /proc/driver/nvidia/agp/status says that AGP isn't enabled. But in mode 2 video runs way faster than in mode 0 (no AGP), so agp seems to be on . Maybe sombody knows why?

My system:
Slackware 9.1
Motherboard: ASUS A7V8X-X (KT400 North Bridge, VT8235 South Bridge)
Video: MSI NVidia GeForce 4 MX 440 64MB AGP 8X

Hope this helps someone!
Marcelo

post your Xf86config file, maybe we can see something else happening.