According to the manual, if you want 32GB, then yes, you will need to ditch all existing RAM and get 4x8GB QUAD-RANK (4R) RDIMM's.  You can use 1333MHz DIMM's, but they will operate at 800MHz, so don't break your back - or the bank - trying to get 1333MHz memory.
If I have 8GB (2x4GB) 1333Mhz Dual Ranked RDIMM in the machine currently and looking to get past 16GB. Can I add any of the following: 2x8GB 1333Mhz4x4GB 1333Mhz Or would I need to ditch everything and get: 6x4GB 800Mhz4x8GB 800Mhz Thanks for any input!
RDIMM Memory module compatibility on PowerEdge T310