James Edelen wrote a nice posting about stresstesting Sharepoint:
"Testing SharePoint is and incredibly important part of capacity planning. There is nothing worse than spending weeks or months implementing a solution and it not being able to handle the required load. Using a tool like ACT will enable you to test your hardware before production in order to ensure that it will perform as expected. It will also let you more easily discover bottlenecks in your system before going live. You can watch performance counters on your servers while stress testing to find bottlenecks."
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