http://www.opensourcecms.com/index.p...d,10,7537,7563
has some bench marks you can see to evaluate speed and memory of each system.

Personally, I recommend punBB for small 10K user systems. phpBB for programmers and vB for people who have no idea what PHP is.

Also, http://www.big-boards.com/ tracks some of the largest forums and what forum technology they use. It is a great place to start.