Originally Posted by
matsp
Since nearly everything to do with the forum is database work - the only bit of a PHPforum that is even remotely CPU intensive is the processing when posting/editing/quoting a post. This is so that the HTML-ization of the post can be done ONCE, rather than doing it every time a post is DISPLAYED (which, for normal use, is probably a factor of 10+ more frequent).
The reason for needing more servers is probably a case of having enough RAM to cache the queries, and to have fast enough disk(s) when there isn't enough RAM. Since a forum like this, with a few 100k posts, hundreds of users, can't realistically fit in reasonable amount of RAM, it's likely that the need for a bigger database server is based on the I/O speed rather than the CPU processing ability (although I'm not 100% sure of that).
[1]I'm slightly more familiar with PHP forum, as I have worked with a webmaster to extend it slightly, compared to vBulletin that this forum uses, but I expect it to use a similar approach.
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Mats