Load testing methods and strategies for webapps could fill a book. In fact, it already fills a number of them plus at least one book on testing SOA and Web Services http://www.amazon.com/gp/product/0123695139
The short short short version is that there are two basic types of load tests that you'll need to put together. The first test simulates real users and includes significant pauses between page hits that reflect how long you think an anverage user would spend on each of the pages. The second simply pounds your site into submission by repeatedly hitting a feature or set of features until they can't handle any more. These tests, while entirely unrelistic let you know where your app's breaking point is.
It's important to remember when simulating users that you have to stagger their hits because it's highly unlikely that your maximum expected number of users are all going to hit your site in the exact same instant. Even better, randomize the pauses and starting point for each of those users. Then rerun the tests a few times to get an average.
K. Rhodes 2007-05-18