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I am using just 667mhz on my 2008 Mac Pro from OWC. At that time they didn't have stocks of 800mhz and I was fine with 667mhz.
 
They can work but will run at 667MHz instead of 800MHz that your computer is capable of.

Can you use 8GB of 800MHz in riser A and 8GB of 667MHz in riser B?

Also would all RAM run at 667MHz or would it work at different both clock speeds?

If 800MHz for riser A is this RAM loaded (used) first (or what is the order of use)?
 
Do you mean a pair of 800MHz RAM in riser A and a pair of 667MHz RAM in riser B? Yes, this would work. All ram will run at 667MHz.

The proper sequence to add RAM is outlined here.
 
I have used it and as mentioned it works at 667 - you won't notice it except in your pocketbook. You can mix and match with a high probability of it working. Install in slots as recommended. Reset pram when you get a chance.
 
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