You can but a 2GB and 4GB on each processor but not 2x2GB on one and 2x4GB on the other - memory configurations for one processor MUST match the other.
I have a PowerEdge T410 with one processor (Intel Xeon E5607, 2.26GHz, 8M Cache, 4.80 GT/s QPI, 4C).  To add a second processor, previous threads have indicated that I have to purchase a processor kit which should include the processor, heat sink and (maybe maybe not) thermal grease.  I also have 4GB Memory (4GB Memory (2x2GB), 1333MHz Single Rank UDIMMs for 1 Processor LV) I understand to add the processor I'll need to add 4G of memory for the new side. My current memory is 2 2G.  Can I get 2 4G and put 6 on each side (a 2G and a 4G for each processor) or do all the memory modules need to be the same?
Add second processor to existing PowerEdge T410 - memory question