Guidelines for asp.net website loading estimation?

I wonder if there is any good guideline for system administrators to calculate the resources needed to host an ASP.NET website? I want to do some estimates on the processing power needed so I know what kind of VPS/Cloud I need to get.

From stackoverflow goodwill

• Load testing will be your best solution.

  Applications differ in how they scale depending on a large number of variables including types of problem solving, approach to solution and other factors.

  100 concurrent users consuming 10% CPU does not mean 500 concurrent at 50%, it could be 20% or it could be 100%.

  Your bottleneck could be processor or memory or network bandwidth.

  Without actually placing the load on your application, any estimation will be merely a guess and will quite likely be proven incorrect.

  goodwill : yep I know, but you still need some kind of estimation to budget for it. Measuring it in the environment is different type of thing
  From Samuel Kim

• Totally depends on the application.
  
  If your looking at requirements for a specific application, then put it on whatever hardware you have that looks like a server, start perfmon running with a good selection of counters & then run some automated tools for load testing, performing some common tasks against the application. See how it performs, then take a guesstimate based on the hardware you have, the load you ran against it & how it measured up .
  
  From experience the bottlenecks i see appearing with ASP.NET apps at the webserver end is usually RAM.
  
  It's the database that runs the back-end of the ASP.NET application you need to worry about :-) I'd say 9/10 times I've seen performance issues with ASP.NET apps, the DB has been the bottleneck.

  From nkav