This is what Intel calls an "unbalanced" memory configuration.  It is a configuration that technically works and is allowed (you are able to boot and use the system) but can significantly decrease performance (hence the BIOS warning). It wants all channels to be populated evenly - not so much the amount of installed RAM, but the number of installed modules.  Populating slot 3 but leaving slot 8 empty creates an unbalanced configuration across the channels.
Hey, I have a Dell R610 that I just put more ram into and I get the message on boot "Unsupported memory configuration". On reboot it says "F1 to continue or F2 for setup", if I press F1 the server boots up fine, all the memory shows up, and everything works great. I have also run the Dell hardware memory test and it came back with no faults so the memory is good too.  So, it seems the server is fine with the new memory, and all the memory checks out as good, but the bios message concerns me ... can anyone enlighten me to what it means and how to resolve it? Note: if it helps this is the original slot configuration if you were to look down at the server from the front: [2G][empty] [2G][empty] [2G][empty]    [CPU]    [CPU]    [empty][2G] [empty][2G] [empty][2G] Now its like this: [2G][2G] [2G][2G] [8G][empty]    [CPU]    [CPU]    [empty][8G] [2G][2G] [2G][2G] Thanks,
Dell R610 "Unsupported memory configuration" bios message