I have the exact same problem :)
I was searching for something unrelated for the ASUS K8V-X SE motherboard when this thread caught my attention. I have the same hardware, an AMD64 2800 and the ASUS K8V-X SE AND a Geforce 6800. Wow, what a coincidence. Indeed, my X does do a complete lockup also. However, SOMETIMES it actually works! It seems to be totally random to me. If I can get the X server started, everything works.
The sad thing is that I actually had another motherboard and I got rid of it because it didn't work well with the GF6800 so I specifically got this K8V-X SE to resolve this problem. It's sad that I seem to have got more hardware conflicts.
Well, my next CPU will be an Intel Core 2 Duo, so hopefully going back to the mainstream will solve these stupid problems.. *sigh*