Some more data on this situation - a few weeks ago, the machine in question had 1GB of RAM, and AGP mode was working fine with the 4191 drivers and RH 8. Between then and now, I had to install more RAM to run some performance tests for a project. After the project was over, I tried out RH 9 and went back to RH 8 (a different story).
With 4GB of RAM, AGP mode will not work (RH 8 & RH 9), period. I pulled out 2GB of RAM, rebooted the machine and now AGP mode is working again (no changes to XF86Config).
This looks like a pretty harsh bug, but I'm not familiar enough with the innards of the kernel, XF86 or the NV drivers to know where to start finger-pointing. In the case of HPC scientific visualization, which is the application we're working on, having support for AGP with large amounts of memory is crucial.
Wes Bethel - R3vis Corporation