My Dell Latitude D630 takes PC2 5300. A memory website that I use says to use this 5300. What if I use PC2 6400S (2GB stick) in this same laptop. Will it work or not? Is the 6400 mean ghz, speed, or what? What's the S for?
If you want to be sure it'll work, as Shinma said, use recommended memory modules.
Generally speaking though, there's no harm in using a higher rated RAM. It is possible that a higher rated RAM running at lower speeds will have improved latency, but this of course, will vary and depend on the particular RAM in question.
Really, you'd need to know the exact specifics of the RAM you're looking at and compare timings etc.