Here is a good blog about performance and load testing for web applications.
Agile Testing: Performance vs. load vs. stress testing
I am currently working on adding load to an application I have been working for the past six months.
So far I can say that this is probably one of the most time and effort consuming phase of the entire project for me. Lucky I have WebLOAD to create the load scripts and simulate the user for the application.
WebLOAD was easy to get started. Initially I could not getting it work under Windows Vista but when I had my new machine with Windows XP it worked like a charm.
WebLOAD can connect to the web server and collection metrics. It also shows you a nice graph of all the stats collected.
When adding load to the application I was able to identify potential issues what would come up once we bring the application live.
Where I am having some difficulties is identifying the state of the application. I am trying to identify what is an acceptable response/performance so that I can say "The web servers can support x number of concurring users" Or "If you want to support x number of users" here are the specs you need for the web servers.
If any has a good experience with loading testings such as what are the key metrics to collection, please shared them with the public.