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For this build I'm going to go for registered memory for this build and throw 24GB into the system in the form of 6 x 4GB modules from Crucial (part number CT3KIT51272BA1067). These come in 3 x 4GB kits so you will need two of them.
Unsure about how much RAM you need? Check out "How much RAM do I need?"
Price: 2 x $105.