How do you upgrade the RAM in the "Early 2009/Nehalem" Mac Pro? How much RAM of what type does it support?

Just as it is for the original and "Early 2008" Mac Pro models, upgrading the memory in the "Early 2009/Nehalem" Mac Pro is quite simple. Even though the memory used in the "Early 2009/Nehalem" Mac Pro models do not have the same enormous heatsinks as memory used by earlier Mac Pro models, it nevertheless is critical that the memory meet precise standards to function properly.

The "Early 2009/Nehalem" Mac Pro models use "1066 MHz DDR3 ECC SDRAM", but more specifically must conform to the following:

• PC3-8500, 1066 MHz, DDR3 SDRAM UDIMMs
• Error-correcting code (ECC)
• 72-bit wide, 240-pin ECC modules
• 36 ICs maximum per ECC UDIMM

Please note that the low-end Mac Pro "Quad Core" 2.66 (2009/Nehalem) has four memory slots on a single "slide out" processor tray officially supporting up to 8 GB but third-parties have discovered that it actually supports up to 16 GB. The high-end Mac Pro "Eight Core" 2.26 (2009/Nehalem) has eight memory slots -- four on each of two "processor trays" -- supporting up to 32 GB.

Apple covers installing memory in the "Early 2009" Mac Pro models on pages 41-58 of the PDF User's Guide -- complete with drawings -- and this absolutely should be read before upgrading the memory.

However, site sponsor Other World Computing noticed that the manual and the Memory Slot Utility software that is pre-installed on the Mac Pro provide contradictory information about which slots should be populated to maximize performance. It's likely best to use the information provided by the application rather than the manual.

Perhaps even better than the official drawings, site sponsor OWC provides a step-by-step video of the upgrade process:

From testing both systems, the always excellent BareFeats discovered that three memory modules in the Mac Pro "Quad Core" 2.66 (2009/Nehalem) and six memory modules in the Mac Pro "Eight Core" 2.26 (2009/Nehalem) are faster than the maximum of four and eight modules, respectively. This is because each Nehalem processor has three memory controllers, so it is slower for the processor to access the fourth one. However, BareFeats noted that the "vast majority of real world applications do not saturate the memory bandwidth" so it wouldn't necessarily be slower to maximize the memory.

In addition to providing helpful installation videos, as you would expect, Other World Computing also has quality memory that is guaranteed to work with the "Early 2009/Nehalem" Mac Pro and that costs much much less than buying from Apple.