As far as hosting goes, more ram would probably be your best bet, versus faster ram. However it all depends what you plan on doing with it, or if you can have both, why not go for that! Latency of the ram plays a roll here as well.
I'd be more concerned with disk IO than ram speed.
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You definitely want to check the specs on your computer (motherboard) to make sure that it is capable of using higher speed RAM. A lot of times they recommend instead of flat out telling you not to use it because you won't see a difference in performance.
IO speed is definitely what you need to worry about. RAM speed does play a role but the faster the IO the better!