I recently put together an approach to really answer the questions:

1. Can my application scale
2. Where are the application bottlenecks
3. What size environment do I need for a target load

I did this because I needed to answer these questions quickly without spending a lot of money for my company. To make the challenge more fun I did not have a large test environment, did not have the skills for performance testing, did not have the skills for modeling and did not have the licenses for either. I also found a way to do this and deliver ongoing performance tuning, testing and modeling services for my company leveraging the initial effort and keeping all those ridiculous assumptions (limited test environment, no skills, no licenses). Interested?