I got excactly the same problem as you, what I did was looking under the XF86 logfile under /var/log/... on the last line I saw AGP4x initializing or something similar. Now the problem is that I only have a motherboard that supports AGP2x.. So what I did was that I wrote in
Options "NvAGP" "0" directly under the
Driver "nv" module in my XF86Config file...
that makes X boot up without using AGP... (which might not be a good Idea in the long run) I mean things run pretty smoothely. But I'm going to see if I could fix the problem.