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.