The chat plugin is great, but it seems to use a lot of CPU and sometimes RAM. I have a chat page and if it's open it will use a continual 40%-50% CPU. If I also open the corner chat box, it will run me out of swap and crash the server if it gets too busy.
I can add more CPU if the problem isn't additive. I.e., 40% if it's one or 100. RAM is a little tougher.
WP MU 3.0.4, Apache with mod_fcgid + xcache.