During the past couple of weeks I have been going across the country with some of the guys from IBm doing a series of events on Exchange Performance tuning... Much of this has been focussed towards the use of SANs, but the information is just as pertinent for anyone running Exchange.
One of the big factors I find out there in many environments is the use of a smaller number of bigger and cheaper disks. Let's face it if we can reduce the cost of deploying Exchange by user hardware such as inexpensive SATA drives, then so much the better. You would think so, but this is not always the case... I would urge you to look at the advantages of using more spindles with faster drives to give greater overall bandwidth, but also to look at the performance of the disk subsystem including controllers as a whole to see if you are really getting the most from your system.
To explain the details there are a number of useful documents and blog posts from the Exchange team that will help and may tools you can make use of...
Optimizing storage for Exchange 2003:
Concepts in disk sizing:
Especially once you deploy Exchange 2003 SP2 on the standard edition and make use of the bigger store capacity...
Details on Exchange 2003 SP2 Std Database Limit Size:
There are many tools to help you look at the performance of your Exchange Server deployment..
Exchange Server Stressand Performance tool:
Why Should you use Diskpar and Diskpart?: