We already noted in a previous blog post that page 56 of the Mac Pro (early 2009) manual may not correctly explain in which RAM slots new DIMMs should be placed, but there is another issue RAM issue too.
While some memory looks like it should work in your new Mac Pro, it may not, because there are specific requirements for memory to meet Apple’s full RAM specifications. Don’t waste your time and money by purchasing RAM that doesn’t meet Apple’s specs.
The screen shot here shows the Apple Hardware Test (version 3A153) error results when incorrect RAM is used in the new Mac Pro.
Apple requires a special EEPROM on their RAM modules that isn’t standard in the industry. This EEPROM and its design requirements add to the module cost, but are necessary for the RAM to properly operate in Apple’s latest flagship machines. OWC’s Memory for Macs is built adhering to Apple’s design requirements, which saves you time and unnecessary headaches both up front and down the road too.
For complete RAM specs and cautions from Apple, read page 41 of the Mac Pro (early 2009) manual, linked as a PDF here.