When looking at your 'top' info, what processes seem to be taking up the majority of the processor time?
Also, what does your mysql.cnf file look like? That's probably the first thing to look at since if it's a stock config or poorly configured on a larger forum that's probably the culprit.
I'd have a look at
mysqltuner. It's a very handy tool that you can download, install and run to help see what configuration variables you should look at.
