What needs to handle 2500 users online at once

[Carnage]

Looking at it, this is one of those instances where you may just have to throw hardware at the issue. A second CPU, more ram or faster drives could help you out depending on where your bottleneck is. (From the limited info you've given us, I suspect harddrives will give the biggest win)

Before doing that, I would highly recomend looking at mariaDB as an alternative to myisam and switching fully over to nginx + php fcgi will help save some resources as well.