We are trying what's up gold's new sql monitor plug in.  It allows me to monitor basic sql server statuses and also sql alerts.  So far it works great, but it does not handle named instances yet.  This fact will keep us  from implementing it now, but they promise a fix soon.  I liked the fact that this would integrate into our existing monitoring solution.  We have been really happy with What's up Gold for our other monitoring needs.  The sql tool looks at the sql services, page reads and writes,transactions and cpu utilization.  You can set notification thresholds on all of these items.  I f you are using what's up gold I would look at the new sql plug in.