Speed needed to run my Xbox 360 and Netflix concurrently without any problems?

At my old house, I was plagued by my sister having a movie addiction and always lagging me off xbox. Now that I am at a new house, various companies are offering me deals on internet. What speed (up/down) is required so that I do not experience any lag while my sister is watching Netflix? I'm wondering if I should just upgrade my ATT or risk the rental fees of the comcast triple play