Software

My total number of users is small compared to yours but you may be able to extrapolate some numbers from ours.

At the lower end, we have a Compaq Proliant 1500 with a P75 and 128meg of RAM that is currently supporting Exchange (v5.0) AND file andprint sharing for almost 100 users.
It is huffing and puffing and scheduled for replacement soon.

At the upper end, we have a Compaq Proliant 5000 with quad PentiumPro 200 processors and 500meg of RAM supporting almost 1000 users for Exchange only (v5.5) and a Compaq Proliant 5500 (dual P450-II) with 2gig of RAM supporting about 150 users and all the bridgehead duties for a 7-site, 8-server system. Both of these servers have average CPU utilizations in the 4-8% range and performance is excellent.

Compaq currently makes a clustered server unit that was designed with Exchange in mind. Based on my experience with our existing servers, I think it would easily handle 2000+ concurrent users but [standard disclaimer] your mileage may vary.

I'll be interest

You would want to look at a few other factors such as growth, average mailbox size, and what kind of users they will have (#msgs/day, size of msg, #contacts, etc). I will assume 10% growth annually, 10MB mailbox size, light-to-medium users.

On the low-end, I would recommend 2 clusters splitting the org. The reason for this is to lower maintenance requirements for large stores (ie backup/restore, virus scanning, drive space, eseutil and consistency adjuster). Each cluster should be made upof 2 servers with shared storage (fiber RAID array) in an active/passive configuration. I would recommend Vinca CoStandby over Microsoft clustering. Your requirements can be made with a dual-733/800 with 512MB RAM, mirrored local drives for system. The shared storage would consist of two 9 or 18GB drives mirrored for logs and three or more 9 or 18 or 36GB drives for the stores.

On the high end, get quad-processor servers with 1GB+ RAM and similar storage to above. I don't have experiencewith the Compaq clus