Software sizings: both art and science

Ive started performing Software Sizings for our clients recently. This is where given a set of metrics and business objectives, you estimate the amount of hardware required to support a given (WebSphere) workload over a certain topology. Sounds simple, but the devils in the details
Lets illustrate with an example.
Customer A wants to deploy an application thats not yet developed and would like to know how many servers/processors they will need to support it in production. There are a set of questions to be answered before we can even start:
What topology are you considering for deployment (single tier, two tier, n-tier)?
What edition and version of WebSphere Application Server will you be running on?
What technology platform will you be using (Windows, Linux, iSeries, pSeries, zSeries, etc)?
What Pattern for e-Business best represents the type of deployment considered?
Will authentication be required? (impact on processing requirements)
What is the desired processor utilization rate?
What is the anticipated transactional volume, and what is the nature of that volume

http://arseneault.ca/blog/2005/08/26/software-sizings-both-art-and-science/