PHP/Ajax realtime updating

[prosportal]

I'm by no means an expert when it comes to Ajax so I'm looking for support here. I'd like to display how many people are logged into our site using PHP, but have it update in real time. The query is simply SELECT isLoggedIn WHERE isLoggedin != 0 then returning the $query->num_rows;...

Also, how much stress would this put on my server? Maybe it could just update every XX seconds or so.

Any ideas?