Q:

Why my RAM runs at 800 MHz in 4 slots of my mac pro instead of 1066?

I've got a Mac Pro 4,1.
I had two KVR1066D3Q8R7S/8G planks in it and it ran on 1066 Mhz.
Now I've bought two planks of KVR1066D3Q8R7S/8GI (the same with "I" at the end - stands for intel certification). I've got 32 Gbs, but the system says they run at 800Mhz.
Why? Is there a way to fix it? Is it because I use four slots instead of a triple setup? Will 3x8Gbs perfom better than 4x8 or is it better to have more RAM?
UPD1:
If I use either three of the sticks, they all run on 1066, when i insert the fourth one, everything drops to 800 Mhz.
UPD2:
That's what I have found here:

Single-processor 
  computers have four 
  memory slots. Depending 
  on the model, you can 
  install 1 GB, 2 GB DIMMs, 
  or 4 GB DIMMs. 
  The processor’s memory 
  controller has three 
  memory channels. DIMM 
  slots 1 and 2 have their 
  own channels; slots 3 and 
  4 share a channel. 
  For optimal memory 
  bandwidth, all three 
  memory channels should 
  be used, and memory 
  should be balanced across 
  the three channels.
Note: Populating slot 4 
  slightly drops maximum 
  memory bandwidth, 
  but depending on the 
  applications used, overall 
  system performance may 
  benefit from the larger 
  amount of memory.

But it is still confusing. I thought the bandwidth that is being described is overall bandwidth not like all chips are dropping to 800 Mhz... 

A:

I believe this issue is caused by the fact that the Nehalem based Xeon processor you have is one of the "Triple Channel" variants, meaning that when you install the fourth stick of RAM you are forcing the memory into the non-channel diversified mode, similar to using 3 sticks of memory in a dual channel system.
This is supported by the fact that you're experiencing the issue only on the 4th stick as well as a fairly large amount of of the supporting documentation surrounding the MacPro and the processor itself.

