The Extended-Memory management feature of the UCS C250 servers enables them to configure up to 384 GB RAM on a 2-socket based server. Dell has with the R810 servers that can handle up to 512 GB RAM and has a feature that enables it to use this amount of memory with only 2 of the 4 processors installed, using it's FlexMem Bridge technology (32 banks).
Dell however is very clear about supported memory configurations: it's complex and only certain configurations lead to using the full bandwith of the DDR3 memory modules: up to 64 GB it will run at 1333 MHz, there above at 1066 MHz. The R815 (with AMD proc's) doesn't have the FlexMem Bridge, but will do 258GB at 1333 MHz and 512 at 1066 MHz.
Primary question is: will the UCS C250 M2 run the memory allways at 1333 MHz, no matter how many you put in it? Ofcourse all the same DDR3 modules, spread over the banks as described in the manual.