Asrock 970DE3/U3S3 + Gigabyte GTX260

Hello,
Just built a medium spec gaming rig for my missus..specs as follows..

AMD FX-4170
AsRock 970DE3/US3S
8GB G.Skill Ripjaws F3-10666CL9D
Antec 750watt HCG-750

Built originally with a corsair GS650 PSU...

Built system, initial fire up it sounded like the PSU may have been surging, also the PSU had blue led's which would dim occasionally with a buzzing sound from the psu...not a good start.

Anyhow, system fired up but no screen or post. I did the usual checks, plugs cables etc. tried again and no go. Luckily I was also making a PC for a mate so swapped out the RAM for the kingstons that were going in his machine in case that was the problem. Still no good.

So, i had a couple of spare video cards laying around, so i popped in an old 8400GS, and it fired up ok. pulled it out, tried the 260 GTX again, no good.

Tried a 7800GT with 1 pci-e connector, and it fired up, took it out, put the gtx back in and once again it failed. Now both GTx's are ok, tried them in another machine to make sure.

So I took the PSU back and exchanged it for the Antec thinking that was the problem. Installed the new Antec and same old story..old cards work ok, but the gtx's just wont work.

Any ideas would be apreciated.. I'm beginning to think the mobo is stuffed...