I've had to deal with this very issue on a number of small networks. Netflix is going to kill your bandwidth, as are torrents. I use IPcop on an older computer with a Layer7-modified kernel to block torrents. Unfortunately that does not block encrypted torrents, but that is easy enough to deal with as all torrents are UDP, so simply capping UDP will put a lid on even encrypted torrents. Netflix is a pain. Their movies are generally streaming from Limelight networks. You'll need to block those sites. Google "blocking Netflix" for more details. I can try to post more details once I get in front of one of the IPcops. Also, Amazon is streaming movies now, and even YouTube can hose your bandwidth.

Parkinson's Third Law: usage rises to meet bandwidth.