You should also make sure that you don't have any artificial limits on the OS. For instance, by default a process can have 1024 open files but that can be increased (see ulimit). That being said, since you have that many users, you probably want to look in to adding additional AOLServers (even on the same machine) instead of upping the one server. The reason for doing that is in case you need to update a file/config then you can easily take one down, test it off line then bring it back up and take the other one down which will provide a seemless experience for the user. When looking at your system, try to see what bottlenecks you encounter when you are around 250 users...it could be AOLserver but it could also be that the OS is telling the process to not take up too much resources.
