I need to enable this mod_xsendfile, i tried google but i can't find anything about how to enable it in dreamhost.
Did you try search in this forum? There are alot of results, but the post your looking for is this one: http://discussion.dreamhost.com/thread-127481-post-148059.html#pid148059
Open a ticket with support via the panel.
It's not on by default (yet); write in to support and ask for it. I'm no longer taking requests for it through this forum.
I got a last question, can i enable it for only 1 domain?