I know the forums which are accessed by 200-300 members by the same time, they have private messaging, captcha, file upload and few of other custom add-ons installed... and everything I could say here - it only depends on what kind of powerful hardware do you have. Proper server configuration also means a lot. I think, 40 or 50 members will not make any critical difference in speed if you have everything properly configured. It could even work on the shared hosting with no big problems.
Anyway, you won't know it until you try it ;-)
And one more thing... Pre-moderation is not for the instant forums with such a lot of members.