Underclock RAM to 1066 MHz?

[strausd]

So I have heard that in some cases when a PC can support 1333 MHz DDR3, there is a point where you can have too much RAM. Where after a certain point, say 4 GB, the RAM will go down to 1066 MHz. I don't know if this is true for all PCs or even for any, I just remember hearing that.

My question is this: Will this be the case for the 12-core Mac Pro? If not, would it be worth the extra cost for 16 GB of 1333 MHz, or would it be better to just get 16 GB of 1066? If it does have the problem, at what point would it go down to 1066 speeds?

Thanks!