I'd love to incorporate alerts into my forum, but my concern is what the load impact might be on my server.  I have a very large forum. Going on 2,000,000 posts. Its not uncommon to have 500 people on at once. I do have a dedicated server.  Is this something I would ned to be concerned with?

The alert system can be automatically disabled when the server reaches a threshold you define, but it's overall really server-friendly and it has been optimized a lot! There shouldn't be any noticible difference when you have it installed.