You need to have enough drive space for your OS, document tree, and log files. In most cases, 2 GB total is sufficient.
Put the OS, swap/paging file, Web Server logs, and document tree each on separate hard drives. If your log files fill up the log drive, your OS does not suffer. Also, you’ll be able to tell whether, for example, the OS paging file is causing drive activity.
Your OS vendor can recommend how much swap or paging space to allocate. Based on testing, Web Server performs best with swap space equal to RAM, plus enough to map the document tree.