I've had a couple of conversations with customers lately, looking for advice in figuring how to plan and size their Exchange 2007 design. In planning for something as potentially complex as a large messaging system, it's easy to get dragged into deep technical discussions around the number of spindles of disk, the IO requirements, how to build the clusters etc etc... but sometimes a high-level overview needs to be presented to get the process moving.
I'd recommend a first step would be to look through some of the excellent posts on the Exchange Team Blog, especially around planning the processor and memory requirements or the disk subsystems. Some of the content from the blog has been published as part of the Planning & Architecture documentation on Technet - some of which provides a great overview of the server deployments required for different scales and complexities of environment.
When it actually comes down to "how many servers will I need?", the question is always a tricky one to answer since it very much depends on where the users are, what they're doing and how much data (and growth of data) you expect to see. HP have recently published some guidance on sizing HP specific hardware, which would be well worth reading if you're going to deploy their kit, but still may be worth a look if you were going to use someone else's.
There's a good whitepaper and a series of useful tools which can take fairly straightforward inputs and give you an estimate (and price, in US$) of what hardware you'll need. They even have some "off the shelf" configurations which are sized at set user levels - of course, it may be more complex to deploy a highly available, well managed environment, and there's no substitute for experience when it comes to designing something like this, but it could be a great start.