We were able to operate more than 20 KDE sessions, each running a different screen saver, on a AMD 1.7 Ghz with 256 MB of RAM. The operating system was RedHat 8.0. CPU should not be a big problem, but, as a rule of thumb, you should ensure a minimum of 32MB of RAM per each session, although we recommend 64MB to play it safe.

The load of a NX server running RDP sessions is less than half than the load of running X sessions. This is because NX server just works as a gateway to the Windows machine where applications are run. You need approximately 12 MB per session, so a 256 MB server should be able to handle more than 20 sessions. We can't actually test this as we don't have enough Microsoft Windows Client Access Licenses.

Please check this article providing information on how to size your server: http://www.nomachine.com/AR11B00075