533mhz FB-DIMM in Mac Pro

And why would you want to use 533MHz FB-DIMMs instead of 667MHz?

Early on there was someone with wrong RAM - and that led to problems, ECC errors. And heat-related. When in doubt, and it is expensive, go by Apple specs. Crucial always does a good job listing what specs, and guarantees if you use their memory selector, but not otherwise.
http://www.crucial.com/mac/

Even Kingston pulled their RAM (though not from 3rd party shelves) before they finally had correct and tested RAM.

Most people buy a computer, then look around for memory. Of course 8GB RAM will set you back almost as much as a Mac Pro, too, but very few need more than 4GB and most can get along on 2GB just fine.

FB-DIMMS are fast becoming the new RD-RAM (rambus), I think in light of AMD and Intel puling back from FB's, prices and availability will be ongoingly problematic.

533's are too slow for the 1333 bus in the Mac pro and are bound to cause headaches.

Lastly it 0seems to be consensus that FB's offer no speed adavantage over DDR2, while I applaud Apple using sever grade components the should have go into xServes only and not Mac Pro's.

You can use them since Dell's Precision 690 workstation line by default uses 533MHz FB-DIMM's (to lower the base cost of the box) and it uses the same chipset architecture as the Mac Pro. Some folks have also accidently bought the slower memory type from Crucial and it worked in their Mac Pro (but wondered why their memory was running slower in System Profiler).

You'll be taking a performance hit since all memory installed (even if they are 667MHz) will end up running at 533MHz and end up slowing down your entire memory system (which is already exasperated by the higher latency of FB-DIMM's). Personally wouldn't recommend going this route.

Mac OS X (10.4.7)