Having some serious memory issues with vBulletin
 

I run a decently populated vBulletin and have had what I've always called "growing pains" as we've gotten bigger. Recently I'm more concerned as I'm having trouble keeping the machine up that is running the software.

I have a VPS from Slicehost that started off as a 256MB (RAM) slice. I have since bumped it up to 512MB, then 1GB, and now 2GB. The transition from 256->512->1024 was decently spaced out. Because of the low memory available, the transition from 1024->2048 happened in about a week. Even now, days after the transition to 2048, the machine is still constantly swapping and essentially running out of available memory.

I've inquired about optimization for MySQL and Apache on these forums and with the information, have not been able to change the outcome. The server is a fresh (few months) build of CentOS 5.2 and only runs Apache, MySQL, PHP, SSH, and Sendmail. We average roughly 300-400 concurrent users with spikes up to 600-700. What kind of memory is required to run the software successfully with these numbers?

Edit: I've attached current "top" output.

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