The Dell documentation is the best starting point.  Both the manual and the spec sheet lists PC2-3200 (same as DDR2 400MHz), Registered ECC ... that is what you should be looking for. Memory will usually communicate at the speed the memory controller (motherboard) is capable of, so memory designed to run at 533MHz will clock down to 400MHz.  This ability is not infinite ... for example, 800MHz may not or may clock down far enough to communicate effectively at 400MHz.  You can usually go a step, sometimes two, above the stated specs, but you should always start with the listed specs.  This is why people can "guarantee" it will work ... and usually because they have tested it. Yes, the FSB does play a role in supported memory speeds, however, all processors certified in the 6850 will be capable of running the memory that meets the specs that Dell has published, so there is no math or research needed.
Hi,  I recently purchased a PE 6850...  I'm shopping for parts for it... I am (and have been since the 1990's) confused when it comes to choosing memory. The specs for the 6850 call for "144-bit ECC registered PC2-3200 DDR2 SDRAM DIMMs, with two-way interleaving, rated for 400-MHz operation" Of course, I buy my stuff used on eBay - so, it is kinda up to me to find the right stuff. Some websites sell 'guaranteed to work' memory for the 6850 that is PC2-5300... As far as I can tell, my (2) cpu's are 800mhz fsb... which I think plays into the choosing of memory. Anyway - is there any current documentation as to what memory I can use - or any advice?  It is hard to search for such information, since you get 1000s of results of sites trying to sell you memory. (I'd search the forums here, but search doesn't seem to work right now)
Choosing Memory for PowerEdge 6850