Well that is dependant on the profile of the user; and difficult to answer. If they are using the advanced Ajax Web Client then most of the processing will be happening on their workstation (as it is a web based technology). The back end grunt work will be the processing of mail eg. av/as scanning, handling the mailstore, zimlets, tracking changes. etc etc etc. The more processing power you can give the better the experience, but the specs above I believe would provide a solid system. Dependant on how you are going to grow you may be better considering a small SAN now instead of later.
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