You basically asked 2 questions. If you're asking if adding a 2nd subwoofer will benefit your movie experience, YES. It might help even more than switching to 7 channels if your room is small (like the others said). If you have a receiver that has 7.1 capabilities, why not borrow some speakers from a friend to try it out and then if it sounds better to you then purchase 2 more speakers of your choice.
Like I said before, if you have 2 subs (even if they aren't the best out there) you will get a better bass response in the room, and better balance in the sound. Movies do really well with bass and having that presence or fullness helps more than having more channels.