I've got the same Board with the X3460 and 8GB unbuffered, Non-ECC-RAM and got the same problems. I couldn't get any HD5xxx-graphics running. At this time i have a poor 4770 running, but it crashes sometimes with the same message. No BIOS-Update was useful. The crashes stop at using Vista, but my HD5870 does not even let a beep out of the board on power-on.
I'm realy dissatisfied with this board.
Another S3420GPLC does not work with Adaptecs 8-Port SAS-controller on PCIe. Two boards - two times bullshit... The Intel-engineers did better jobs the time before!
I changed the board, the system was stable, however after I loaded the Flight Simulator, it started crashing. Not the same error though, just verical lines and reboots itself with some 9-10 angry beeps.
Now it crashes randomly, I strongly suspect the GPU card itself,