Yeah to geek out a little on this I have the 2009 8 core 2.66 4,1 model.
It can take 1333mhz memory which I recently installed, but it reads it at 1066mhz. So if in the future I bust my CPU's limit and want a faster machine and find some Xeon's for cheap enough, upgrading the CPU and EFI will get me a 30% increase in RAM speed.
