[_ck_]

Let’s put it this way, if you have decent income from a forum of that size, you should not waste another minute your time (or anyone else’s) and just pay for vbulletin and the extra hardware it will need at that size (dedicated mysql and email servers). Seriously, it’s the best for a reason, but you will have to pay for everything once you go down that path, addons, etc.

If you are trying to cut corners and do it on the cheap, bbPress 0.9 is lightweight enough to handle it (wordpress.org has many times the traffic you do) though a dedicated mysql server can always help, and keep in mind bbPress has no caching plugins available yet. The money you save will be spent in hours of customization and tuning and creating your own fixes for problems.

You can always turn to lighttpd or nginx if you know your stuff and have days to fiddle with it or the almost instant installing litespeed, if you want an apache replacement that will double the capacity of your server.

The only cloud on the horizon is bbPress 1.0 as I estimate it in some cases will be roughly 30-50% slower than 0.9 (with still no caching plugins) and at this time roughly half of my plugins will not work with it (and won’t for quite some time, assume the end of 2009 at the soonest).

After 1.0 stabilizes and someone ports WP Super Cache to it, that will change the picture again but that kind of caching only helps visitors that aren’t logged in.

Oh and bbPress isn’t going to solve your search problems. It will only make it worse. bbPress’s search is completely non-indexed, and has virtual no options. My Super Search plugin might eventually cache searches but that’s not going to solve the problem of 1000 users searching for different things at once. I mentioned http://sphinxsearch.com a few months back and amazingly they are now using it on wordpress.com so there is hope that one day there will be a plugin for wordpress/bbpress to use it for searches on larger sites which will completely solve the performance problem.

Your groups of forums will require bbPress 1.0 and the downside of that again is half the plugins for bbPress 0.9 will not work with it. If you do install 1.0, don’t install anything more than 1.0 alpha 5 to use with my plugins.

ps. the top 100 list hasn’t been updated in 90 days and many of those forums are much bigger now – next update will be sometime after Firefox 3.1 is released