I'd like to re-ask the How to justify a workstation upgrade question that Garry Shutler asked with the context of some of the excellent answers given.
I believe that quantifying the productivity increase (extra working hours) is the most effective way to do this.
My case in point: I have a fast machine at home and a slow one at work. My estimate is that I would gain about 30 minutes a day of extra productivity at work if I had my home machine at work. This productivity would come from less waiting to do all the tasks that I do. (An extra 30 minutes a day is about 3 weeks a year.)
Problem: I need to measure this.
Is there a software utility that can monitor and scientifically quantify time taken by tasks on a machine?