Over the years I have seen this question pop up pretty consistently, and that is the "Hardware" question. "What's the best workstation for Revit". I'm not sure if you are aware of it but there's a relatively new page on Autodesk's website that helps you find suitable hardware for Revit, and other Autodesk software as well.
There's a good description of it here: Without A Net
There's a good description of it here: Without A Net
With the advent of Cloud computing there are many more options that include virtual workstations, "Private Clouds" and so forth that should be considered an option because they can be easily expanded for little cost and is considered a "Capital Expenditure" which can be billed to a job, versus the firms overhead, or your pocket book.
A rookery of folks on the LinkedIn Revit Users discussion board commented on the question of hardware and provided a slew of opinions for you if you are interested. This rookery included a few folks from BIM9 and Advance 2000, who offer cloud services.
Cheers
