您如何“指定”Web服务器以支持给定的应用程序？

I am having real trouble trying to get decent answers to my questions from VPS and dedicated hosting provider's sales people. I have a fairly simple set of requirements - how do I select a server spec / package and be confident that I have got it about right? Are there metrics to use - number of Http requests per minute for example? are there other benchmarks? How do you approach it?I am having real trouble trying to get decent a