if you're asking in here, ten to one you should probably try and get a hold of a few consultants that have done this before.  performance evaluations are not that easy to do, in my years as a consultant I've done several of some of the largest ones, some of them down in HP Labs in California.  They took months to prepare.  However,  Internet tech such as this is usually not so involved, but as the saying goes - you can make stats say anything - you have to know your assumptions and set realistic performance goals and make them "real world" prior to initiating your tests.  ie: what is the concurrent user total you need to hit? the average think time of each user? are the users just reading or updating data?  approximate ratios between users updating data and reading information and so on and so forth.  Then after you run that over, talk to your client (unless of coures it's you) then you simulate the load and see how your system does (both hardware and software) and adjust accordingly until you have a solution that fits your overall assumptions within a stated percentage.