Well, Let me explain you little about Databases in DreamHost.
Yes, you can have a database of 200-400 MB with DreamHost, even you can host 30 phbb forums on single hosting plan if you want, but your CPU resources must not be exceeded 60 Minutes per day.
It depends upon your forum traffic.
CPU minutes is the time Server takes for a php page to be generate.
71 US $ Promo Code = DESIWEB2578
Yes, its 71 $ Discount code.