I don't have any image on my display anymore, I haven't done anything unusual, yesterday everything was fine.

My display doesn't receive any image (if I take off the cable, no signal message appears).

My system, with no image on display: I5 2500K, 8GB RAM, Seasonic S12II 620W, Gigabyte GA-Z68AP-D3, Win 7 64, Radeon 7950 (requires 2x6pin connector)

My older system: Intel Core2Duo E6400, 2GB RAM, Gigabyte GA-965P-DS3, Spire Rocketeer IV 600W, Windows XP, Nvidia 8800GT (requires only 1 x6pin conector)

What have I done so far:

took off each of 4GB RAM, nothing
I have image if I connect my TV to the HDMI onboard videocard
with the older card Nvidia 8800GT, installed in the new system, I have image, with each of the two 6pin cable connected
put Radeon 7950 videocard in the older system , I have image!!!
I removed the Seasonic power source and replaced it with the older Spire source, no image with Radeon 7950, but the older 8800GT has image. Lated edit: Radeon 7950 has image in the PCI4x slot, but not in the PCI 16x slot

What could be the problem?

Edit: both video cards display image on the PCI4x slot, but not in the PCI 16x slot

Seems to be a problem with the PCI 16X slot, but only with Radeon card. 8800GT works fine.