What is the difference between 3 Megs/sec and 6 Megs/sec? (Besides 3 Megs)

[EdWood1]

Thinking about going with Comcast "Triple Play" at $99/Mo. for 12 months or the down graded version at $79/Mo. for 12 months. The difference is 3 megs/per sec faster and unlimited calls within the U.S. for the Triple Play. What I am trying to find out is: What am I limited to with the 3 megs/sec compared to 6 megs/sec?