RAM is MySQL's best friend
Really, at minimum you should have 1GB of ram, and with 200 users online at a time, having more would only be better. RAM is cheap, so that is the best place to upgrade before upgrading elsewhere.
Also, what version of MySQL? That is very important as well. If you aren't using version 4.x, you really should consider upgrading as it will increase performance a ton as well bring server loads down.
