Some of you will remember we had a session on this a while back at one of the meetings.
Wel in case anyone needs it, here is a link to a great resource on how to calculate storage requirement (IOPS etc) for Exchange 2003.
http://www.microsoft.com/technet/prodtechnol/exchange/Guides/StoragePerformance/fa839f7d-f876-42c4-a335-338a1eb04d89.mspx?mfr=true