I checked the CPU pins when I purchased the motherboard, and they are fine. The card is installed OK, and lights up briefly red when I power on (I assume it would flash or not light up if it was not connected properly). The card worked perfectly on my previous motherboard.
I've tried re-seating the 4850, to no avail.
If I could get hold of a different Gfx card and try that, then I could isolate whether the motherboard is the cause or not..