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I might be wrong but i thought the old mac pro memory wasn't FB-DIMM but just regular ddr2 memory?
If that's the case i don't think it would even fit in the 2008 mac pro DIMM slot.
 
Thanks, all.

Apple is replacing my old mac pro with new one and I have 11gb of old ram and new one coming with 8gb, wondering if I could fill remaining 4 slots without having to buy new memory.
 
As I've said before 667MHz FB-DIMMs will work fine in the 2nd gen Mac Pros but will cause all the RAM in the machine to run at 667MHz instead of the optimal 800MHz.

I know it works because I did that when I first transitioned from 1st gen Mac Pro to 2nd gen.
 
Thanks - that's exactly what I wanted to hear. 1st hand experience always trumps the internet experts who repeat what they've read somewhere.

The barefeats reference was also very helpful - I'm actually quite shocked how little an impact it seems to cause - from 0 to 4% hit.
 
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