I have this motherboard http://bit.ly/cMQ2kH and this video card http://bit.ly/cUTLkS
CPU-Z and ATI CCC indicate it is running at 4x instead of the full 16x. I have no other pcie cards installed there is no setting in the bios except which video to init first and it's set correctly. Is it possible that having too weak a PSU could cause this?
Yup, thats one possibility due to the fact that the bandwidth options are directly related to how much power can be delivered to the slot. If its not getting enough to run at full speed it will switch to something it can manage.
This is only one possibility. The card its self could be faulty of course. You might be missing an additional power connection to an empty port on your mb... stuff like that.