Matthew.Ryan,  Unfortunately with UDIMMS installed in the server, it is currently maxed out. This is due to the memory restriction that dimm population for UDIMMs and Quad-rank dimms is two per channel. So the 3rd dimm per channel won't be supported while using UDIMMS.  Let me know if this helps answer the question.
Hi,  My server previously had 1GB Ram modules in A1 A4 A2 A5 A3 A6            B1 B4 B2 B5 B3 B6  I am looking to fill all empty slots with 1GB modules however when i have done so it flags up a memory error and will not boot. I have looked in the BIOS and the memory configuration is set to optimized (It will not allow me to change this). This also has two CPU's - FYI  Thanks  Matt
Memory Upgrade - Dell R710 with 1GB Dimms