Ever wonder how many concurrent users your website can support? There are several options for load testing. Depending on whether or not you need third party certification I would suggest starting with one of several load testing applications. Most are client based and can simulate significant loads on a website. You can download these tools (after purchase of license) and run them yourself. By running them yourself you can tweak the site to meet the performance objectives (little changes can have a big impact). It is really a smart way to build your websites (i.e. testing as you go). Next, if your customer requires a third-party test we (assuming you are our client) can run the same software test and provide you with a letter attesting to the result. Finally, there are other companies who specialize in load testing for a fee (I would pick this option last). Does this help? Here are a few links to load testing software:

http://www.webperformanceinc.com/
http://www.empirix.com/
http://www.loadtestingtool.com/
http://www.softwareqatest.com/qatweb1.html#LOAD