Does your living room open up to hallways or a kitchen or dining room? That will play a large roll in the impact of your sub.
Out of all the speakers you buy I feel you shouldn't go cheap on the sub, especially if it is going to be 80% for movies.
If you are looking for more recommendations there is a ton of information under the subwoofer category on audioholics.
I would spend the money on either diy or go with one of the internet direct companies. This will give you more bang for your buck. Set a limit you want to spend on a sub and grab the best one you can in that price range.
