There's one other thing that can cause your problems but that might be fixed in your kernel version. (but I don't know)
There is/was a bug related to AGP on AMD systems in the linux kernel. I won't explain what it really was but it caused a lot of instability. As a workaround to that bug you can add the following line to lilo.conf: "mem=nopentium"
(after it run lilo ofcourse)
That might fix the problem else it is very likely something in the kernel.
And your mobo really supports 4x AGP, but because you set it to 2x in the bios 4x doesn't show up.