667 MHz v 800 MHz RAM in a Mac Pro 3,1 (Early 2008), will it make much difference?

In this case and Early 2008, 667MHz FBDIMMs have never shown to be an issue, work fine (4% hit at most, which can also be offset by using all 8 DIMM slots with four sets which improves memory bandwidth to it s best).

There have been a few 'enterprising' individuals that have had access to and used 8GB DIMMs for 32 and 64GB total memory w/o trouble but in that case you would want 800MHz DIMMs.

http://eshop.macsales.com/shop/memory/Mac-Pro-Memory#800-memory

FBDIMMs like many memory modules are not locked to a specific timing and frequency and can be backward compatible.

These from Amazon are only 2GB DIMMs but known to work in 2006-2008. When you get into 4GB the price has jumped up. And 8GB prices have been in the stratosphere, or had free access to server memory source.

http://www.amazon.com/BUFFERED-PC2-5300-FB-DIMM-APPLE-Memory/dp/B002ORUUAC/

Komputerbay 8GB (2x 4GB) DDR2 667MHz ECC FB-DIMM (240 PIN)

http://www.amazon.com/Komputerbay-PC2-5300F-Buffered-FB-DIMM-Heatspreaders/dp/B0 05HIWD5U/

OWC 8GB FB-DIMM Memory Upgrade Kits for Apple Mac Pro 2008 Released 64GB $320

Ideal RAM config is 8 DIMMs - offers 15% improvement memory bandwidth performance

667MHz FBDIMMs are cheap, excellent and 4% 'hit' you would never know about.

Never use any RAM that doesn't exactly meet the specifications for your model.

The memory controller will just as usual downclock and use the slowest MHz it detects, and it does add to the complexity.

I don't know enough about 'ranking' but that too plays a role especially in mixing different spec'd memory and brands (even different batches of same spec'd DIMM can vary too).

For UK, other than Crucial and Kingston, both usually cost more, otherwise these are ones I have heard others mention:

http://viewitem.eim.ebay.dk/KOMPUTERBAY-8GB-2X4GB-MEMORY-for-APPLE-MAC-PRO-EARLY -2008-31-DDR2-800MHz/140962093586/item?transId=808969265004