With OpsDash, you can easily set alerts for upper and lower limits for any metric. Both critical and warning limits can be set. Just click on the graph in your OpsDash dashboard, and enter the appropriate values under alert settings.
OpsDash alerting has been carefully crafted to prevent notification spam. Notifications are sent out only at the intervals you specify, summarizing the ops situation at that point. OpsDash deliberately does not send out per-event or per-server emails.
OpsDash comes with powerful but easy-to-use notification routing and suppression features. Find out more about how to set up alerting and notification in OpsDash here.