Which performance counters can you look at on a SQL Server Instance to determine it's performance and over all health?
Here's my Perfmon tutorial for SQL Server: http://www.brentozar.com/archive/2006/12/dba-101-using-perfmon-for-sql-performance-tuning/

For more counters and thresholds, here's a poster we did when I was at Quest: http://www.quest.com/documents/landing.aspx?id=11635