Just a note for anyone else out there who wants to upgrade the RAM in their Nehalem Mac Pro, faster RAM will correctly clock down to the 1066Mhz that our systems run at. The 1066Mhz RAM is often more expensive than 1333Mhz RAM and after extensive searching with Google I couldn't find any other posts of people who had tried the RAM out. So I thought I'd be a guinea pig. I put 2 Kingston ValueRAM 2GB sticks in my Quad Core 2.66Ghz Mac Pro (2009) along with 2 of the original 1GB sticks and the system is running much better now with the 6GB of RAM. For a $70 upgrade, not bad.