My Epox 8RGA+ didn't need to have the onboard disabled when I installed my GeForce3, and when looking through the BIOS there did not seem to be an option to disable. I imagine the nForce2 checks for signal in the AGP slot, and if no signal is present, it defaults to onboard.
Do you have another AGP card available to you to test in the AGP slot? And as rastorize mentioned, make sure to disconnect anything related to the onboard video.