Firstly, why are Eclipse not fixing this problem??
Your 8600gt is certainly PCI-express
Are you saying you've had to switch to using onboard graphics, and connected the monitor lead to the video socket on the back of the motherboard rather than the one on the graphics card?
Is the graphics card still installed? (most mobos will detect the graphics card and disable onboard graphics)
I'm not familiar with the bios on your pc. On mine, the terminology is something like 'primary display adapter'