Graphic card/Motherboard problem.

Hello there smart people!

PC1 (MSI K9N Neo V3 motherboard, MSI Nvidia GF 8600GT Graphic card with 2 DVIs)
PC2 (Gigabyte GA-MA78GM-S2H motherboard, Gainward Nvidia GF 8600GT graphic card)

PC1 worked fine for 4 years...
PC2 is perfectly fine, only used for testing purposes.

PC1 starts up with 8 short fast beeps (that means some kind of problem at the graphic card, since I have AMI BIOS), and there is nothing on the screen. Windows starts normally if I wait, I can hear the logging in sound. From here I tried to find the problem.

I've put in PC2's Video card into PC1, it worked... here I thought, that the other video card is dead.
But then I've put PC1's video card into PC2, and it started normally as well...
I've borrowed another video card, and tried it with PC1... worked as well...
So the problem is, PC1 only has problem with it's own Video Card, and I have no idea why.

My question is, what can I try to do, if I want to start PC1 with it's own video card. Swapping them is not an option since MSI doesn't have a HDMI output, Gainward does, and I need that output for PC2. BIOS reset doesn't seem to solve the problem either.

Every help is much appreciated.