Fury - in one of my other personas (yes, I have MPD) I am an administrator of an Invision Board and we have over 16000 members, 12000 topics, and 105000 posts.  It holds up quite well for us, in fact I don't recall it ever crashing.  The one thing I really dislike about it is that we can't ban users with "unknown" IP addresses.
FWIW - the site is 
http://www.simmods.com and I am known as Vince Klortho there. 
 
BTW - Invision has a "service" whereby they will convert all your posts over to their board's format.  The trouble is that the embedded HTML from emoticons and such didn't get transferred worth a damn and they billed us for it.