check what the nvidia driver says about AGP in the /var/log/XFree86.0.log file. It should be using either agp or its own driver. it could say that it can't confirm AGP use. Based on the lack of any info from the dmesg (just loaded driver, didn't say it found any hardware), it's not finding the agp.
possible fixes if that is the case:
- update your 2.6.1 kernel using the x86-64.org patches, for better agp support (something about it being found would be listed in dmesg if it was)
- some boards have better luck finding AGP with ACPI enabled. YMMV.
- maybe tell the nvidia driver to use its own agp driver (X driver config option "NvAGP" "1") if your agp chipset is supported (which i kinda doubt on x86_64 myself).
hope this helps.