Dreamhost is, uh, a dream to work with if you have multiple domains. The control panel makes it so simple.
I run about seven domains from a single account, and about four subdomains. The control panel lets you specify what directory path is the 'root' of your domain, so it works much as you are used to. When I add a new domain I just add a directory with the same name and point the domain at it (for example, /user/myaccount/mydomain.net)
Subdomains allow you to split a single domain even further. Start with mydomain.net. Add forum.mydomain.net and that's a subdomain. Add gallery.mydomain.net and that's another subdomain. You can create an all-new directory with your subdomain name: /user/myaccount/gallery.mydomain.net or split off a subdirectory to use: /user/mydomain.net/gallery/ both work just fine.
I run several phpbb's with no problem. I don't add hacks anymore because I get tired of reapplying them when I have to upgrade to new versions :-/