I run a community on Invision Powerboard with 3100 members and 135,000 posts. I had to get a dedicated server at around 1500 members because I was using up so much resources on my shared host I was using.

With just this site on my dedi, I am using about 16% cpu and 33% memory. As the community grows and I am thinking about opening a couple more sites with one or two having a forum, I wonder which software is best for big communities. Which one runs on the least resources or does it matter?

I look at big-boards.com and most of the huge boards run vBulletin or a custom built script. What would you recommend for forum software?