You'll have to reduce your cpu usage to stay with in accectable ranges. Not sure how long you have to accomplish that - I'm sure dreamhost will give you plenty of warning if they heavn't already covered that info in an E-mail.
As far as I know, cpu used is the same for the various shared hosting packages. If you are really using so many resources that you can't cut it back, some add revinue could probably cover dedicated costs. However, keep in mind that DH isn't offering dedicated machines right now.
I think you just missed it. A 30-40 cpu limit was put in place some months ago. It was then raised to 50-60 a month or so ago. You can turn on cpu usage reporting for each user, and keep track of it that way. This is dreamhost's way of warning you and letting you know that you're using up too much. Rather than charging you extra for the usage, or just kicking you out, they've been good enough to move you to a different server where you can keep operating your site, while you work on reducing cpu usage.
What you'll want to look into is cuting out extra features of your site that people arn't using or don't need. For instance phpbb forums have a lot of extra junk and tend to be resource hogs. You might consider switching to a friendlier forum like punbb. Pages that have a lot of dynamic content can be he couse of trouble too, if you can see about reworking thoes areas so the server doesn't have to assemble the page before sending it out to the visitor.
Hope this helps.