With the nVidia drivers, if you use the internal NvAGP AGP driver, you don't get another module like you would with agpgart. It's internal, integrated into the NVdriver (... well, formerly NVdriver, now nvidia.o) module.
Here's a thought on the problem though -- could it be APM or ACPI related? Do you know what i8kfan uses to control the fan speeds? It may not be a conflict between two modules as much as just an APM or ACPI call that i8kfan uses that the video card module can't handle properly.
I assume you found out that i8kfan was the problem by running it manually or something?