I have Dreamhost PS because I didn't want to risk my sites being down or unresponsive due to the server being bogged down by other users on my server.
I just have a few WordPress installs, a wiki and for a while I had a small phpBB.
Nope, your entire account is moved over. You only get one server, but that server is a virtual host for all your sites.
Note that support has told us that you can have one PS account and one regular account. I'm not sure of the logistics of getting that set up.
That depends on what you think would be more economical for you and also whether you care whether all your sites are running off of one server. Personally, I'd recommend putting all your mission-critical sites on your PS account and have your more casual sites on a regular shared account. Of course, this argument only applies if you want to use PS for reliability.
You really need to either benchmark your application to analyze its performance under a number of different conditions and load. This information will tell you where the performance bottlenecks will be for your application under the loads you expect. If you expect the web server to be your bottleneck, get PS for that. If you expect the database to be your bottleneck, get PS for that.
Note that this only makes sense if you're getting PS for dedicated resources because you expect your app to use more resources than are allowed under a normal shared account.
Use the [color=#CC0000]3DOM50[/color] promo code for 3 extra lifetime domains and $50 off
More Dreamhost coupons here!