Put the OS, swap/paging file, Application Server logs, and document tree each on separate hard drives. This way, if the log files fill up the log drive, the OS does not suffer. Also, its easy to tell if the OS paging file is causing drive activity, for example.
OS vendors generally provide specific recommendations for how much swap or paging space to allocate. Based on Sun testing, Application Server performs best with swap space equal to RAM, plus enough to map the document tree.