In SQL 2005 you can still use sysperfinfo but it is now deprecated and you should start reading data from sys.dm_os_performance_counters if you have monitoring in place for SQL2000 and are migrating to 2005 you should probably update your scripts. One other change you might need to make is to the column that stores cntr_value as this is now a BigInt in SQL server 2005.
This article covers how to monitor performance pre SQL 2005 which is still relevant but attached to this post is an updated script for SQL 2005 It is recommended that you create a new "Performance" database on your server and run these scripts in. Then setup a job that executes every minute to record the performance data.

Update:
It turns out there is a bug in the SQL 2005 user connections counter. The counter is not reduced when a Service Broker task is ended. The result is the "User Connections" counter keeps getting bigger and bigger. SP2 has just been released but this is still an issue I guess MS may fix this in SP3

Script to monitor sysperfinfo updated naming conventions SQL 2005.sql (4.84 KB)