Onboard video won't stay enabled

I’m trying to install an EN8600GT card without using it for display purposes, but my onboard NVIDIA video won’t stay enabled.

I have an ABIT NF-M2 motherboard with NVIDIA GeForce 6150 onboard video, but I am so far unable to locate a setting or control that will leave it running when a video card is also present in the system.

Does anyone have any suggestions or know of a utility that might let me have both video systems running in parallel?

Thanks for your time,

FWIW, I tried to do the same on my computer (Intel X3100 onboard ‘GPU’ + 8600GT). Same problem. The thing doesn’t even want to continue booting when you plug in your monitor cable into the output of the on-board video connector.