Windows Server 2003 Performance Counters Reference
ASP.NET Performance Monitoring, and When to Alert Administrators
To determine the busy sites on a server, there are a few parameters you can monitor. In the System Performance Monitor, select the Web Service object. In select counters from list, select the Total Method Requests item. Then, in select instances from list, select the sites you want to inquire on.
The Total Method Requests item displays how many requests the site has had since IIS was started. From here you can quickly deduce which sites are the busy sites and start planning the log file subsystem accordingly.