I believe pci-e devices are powered by the 4 pin (or 8 pin) power connector, so this doesn't make sense.

Edit:  Also, it seems you've had a short between the 3.3v rail and ground.  PCI devices would utilize the 5v rail.  This doesn't appear to be caused by too many graphics cards onboard.  I've run 4x 5870's severely overclocked in each rig without any powered risers for months straight without any issues.