Building a secure and available system is like forming a sports team that is aiming for the top. It is important to have top players in the first lineup, but it is equaly important to have a "long bench" ie top players who can always jump in and fill any position in the game, during the game.
Exchange 2000 is mission critical system, the one that your company depends on, the one that your job and your reputation depend on and the one where you prove your competence.
So, hard disks are relatively cheap today. It is not worth taking risks which could easily be avoided with one or more redundant disks.
I would go for:
- Mirror (2 drives) for OS,
- RAID 5 (3 drives) for data disk
- 1 drive as hot spare (if you can purchase additional drive)
If I could, I'd go for even more options:
- Rack mounted server
- Additional 431 RAID controller
- External storage rack
- dedicated UPS
Exchange 2000 also enables you to define more storage groups and databases. With this featureyou can separately manage message storage locations for various departments (management, financial, sales etc).
And remember, maintaining an available Exchange system is like a sports season. It is long and you should be well prepared for every situation and you should practice for disaster recovery scenarios.