Like the PowerMac G5, the MacPro requires you to install RAM in matched pairs. Also, the RAM in the MacPro is ECC fully buffered RAM. This is expensive RAM and no other kind of RAM will work in the MacPro. Its recommended that you stick to known good brands as this RAM gets very hot and cheaper brands don't provide adequate heatsinks. Therefore, the RAM has to slow down its speeds to maintain a safe operating temperature. This will result in a loss of performance.

In the spring 2009, Apple updated the architecture to Nehalem. This adds in DDR3 RAM running at 1066MHz. Unlike previous models, they are not fully buffered. Apple has 2 different models with 2 different maximum amounts of RAM. The quadcore MacPro has a max ram amount of 8GB while the dual quadcore has a max ram amount of 32GB.

MacPro

Intel Xeon Processor



2.0/2.66/3.0 GHz Xeon Duo (Woodcrest)
PC2-5300 DDR2 ECC Fully Buffered DIMM
Max RAM 16 GB

Intel Xeon Quad Processor

3.0 GHz Xeon Quad Core (Clovertown)
PC2-5300 DDR2 ECC Fully Buffered DIMM
Max RAM 16 GB

2.8/2x2.8/2x3.0/2x3.2 GHz Xeon Quad Core (Harpertown)
PC2-5300 DDR2 ECC Fully Buffered DIMM
Max RAM 32 GB

2.66/2.93/2x2.26/2x2.93 GHz Xeon Quad Core (Nehalem)
PC3-8500 DDR3 ECC DIMM
Max RAM 16GB (QuadCore) 32 GB (8-Core)