Since the perf bottleneck of many TFS systems will be the SQL Server instance where the databases reside, it's often times the piece you'll want to keep the closest eye on. SQL Server 2005 has some great built-in reports covering many of the usual things you'd want to know, which Thom talks about in this blog post. It includes some screen shots which should help guide people through it and show them the kind of report they could expect.
One of the new features of SQL Server 2005 is SQL Server Management Studio. The reports are part of the built in management interface and are a very nice way to see a variety of statistics about your server.
As an example you can follow these steps.