Gayan has a good article worth reading if you are interested in performance tuning and monitoring of a SharePoint Portal Server.
Performance monitoring helps organizations identify performance bottlenecks of the current environment and identify the right time to scale out the SharePoint server farm. I will be looking at what are the critical performance counters in a SharePoint Portal environment and how to monitor them. This article includes information regarding analyze logs information to determine the best scale out options for SharePoint server farm. Organizations can use the Microsoft Windows Server 2003 System Monitor, Performance Logs and Alerts to monitor the counters described in this article.