I have your newer Xeon processor motherboards and the ram listed in the recommended ram list is KVR266X72C25/1025, the ram we are using is KVR266X72C25/512, and we are using 2 sticks of it, in banks 0/1. Is the 512MB stick not supported, only the 1GB sticks?
Please verify the part number you gave, the part number you listed is for unbuffered memory. Kingston numbering system for registered memory would be KVR266X72RC25. The "R" after 72 means registered. The table below is taken from our web site, notice the "R" after 72 which is the bit width which in this case means ECC.