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What he said, plus 2006-08 Mac Pros need fully-buffered (FB-DIMM) modules with custom heatsinks. More expensive to manufacture.
 
Intel has chosen some memory technologies at times that were needed for then hi end performance that ended up getting replaced by more mass market memory that was fast enough, then later faster.

I remember RAMBUS memory back when the Pentium 4 came out. Then DDR memory got faster and replaced it.

FB-RAM, now we have DDR3, etc.
 
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