The SharePoint community have a number of useful tools that can aide you in your performance testing of MOSS. The tool that comes to mind is the SharePoint 2007 Test Data Population Tool, this tool can bulk load data into SharePoint. If you click on the Releases tab you will see a release: MOSS Performance Load Tests. This is a series of web tests that can be loaded up into visual studio team system:

The visual studio load testing features can simulate multiple users over a period of time all running a defined test mix. In the above image the homepage is hit 9% of the time, whereas the general PageView test which is a broad selection of pages makes up 28% of the test mix. Your test mix will depend on how you perceive your users using the site.
Visual studio has a number of options that you can tweak such as:
- Browser mix – provides a way to simulate multiple browser types i.e. mobile devices, firefox, IE etc ..
- Network mix – your test network might not be the same as production, this allows you to test with network limitations in place
- Load pattern – Constant, Step or Goal based, depending on what your testing, you could hammer your server with a constant load, or you could build up in a controlled manner.
- Warm up time – You can warm the server up to avoid the initial compile times from impacting your results.
In the example above I’m using the test mix from the Microsoft document: Estimate performance and capacity requirements for portal collaboration environments, you can find other documentation for different portal types here.