The only way to get to 24GB while still using your existing 4x2GB DIMM's is to add two 8GB DIMM's (one in each processor memory bank, slot 1) or add four 4GB DIMM's (two in each processor memory bank, slots 1, 2). Slots are numbered 4,1,2,3, starting from the slot closest to the processor. support.dell.com/.../510en.pdf Also, make sure to get memory that matches what you have.  Unbuffered (UDIMM) memory can NOT be mixed in a system with Registered (RDIMM) memory.
I have a R510 with 8GB (4x2GB ECC 1333 MHZ single ranked RDIMMs) with 2 E5620 CPU's.  I want to get the memory to 24GB. I would like to add to the memory and not have replace what is in there if possible. What do I need to get?  I'm confused about the memory configuration with  dual processors.  Thanks for your time.
R510 Memory Upgrade