Web Wiz Forums used with SQL Server 2005/2008 or mySQL will perform very well and will not course slashdotting effect.
We have many forums running on our own shared hosting services with 100 to 300 active users at all times and with databases containing around half a million posts. These forums have very little impact on web server and database resource levels and do not effect other sites running on our systems that share the same servers and resources.
The following larger forums are just a few examples of forums running on our own servers that have larger amounts of activity and do not effect other sites sharing the same resources:-
The Brick-Yard
Popular Volkswagen Commercial, Camper, & Bus Forum
SpeakerPlans
Popular Audio Industry Forum
All Empires: History Forum
All Empires Gaming ForumIf you are concerned about slashdotting then you could always look using one of our hosted solutions and have us host Web Wiz Forums for you. You can setup a sub domain using an A-record in your DNS so that the forum is a sub domain of your main site but running on our systems.