This is the second part Andrew's answer to the question posed in Clustering Notes.
Given the size of your environment, consider an array of cheaper boxes set up in Domino clusters. Think about spreading those thousands of users across several clustered pairs; say 500 users or 1000 users per cluster. Then you can use a bunch of cheaper servers with great big cheap drives. The paired servers provide the redundancy of RAID, the load distribution means smaller machines can handle more people, and the cost stays very low. You may even want to look at using Linux as the operating system of choice for those pairs of servers -- particularly for your DWA users. The Web server response time for DWA on Linux has been significantly faster on the same hardware (sometimes twice the speed) when compared to the Windows servers.
Consider also some ways to reduce the file size per user. Domino does support SCOS (Single Copy Object Store) so if your users are sending around big files to each other, only a single copy of that file is stored; this is instead of one copy in each mail file for each user for every single message sent plus every reply that included the original. SCOS can be tricky, but has gotten pretty solid and in a clustered environment can be a great way to save huge amounts of space.
I hope this helps.