I have 75mbps from Comcast, which allows me to stream from two Roku's at once, plus have two phones and a computer or two, plus a wireless printer connected at all times. I think HD streaming video requires is about 5-10 mbps. The only thing a faster speed would help me with would be actual downloading of content to a computer.
Speed tests show that I get about 25-30mbps via Wi-Fi. Very rarely do I have any buffering problems, and if I do, it's on big days when everyone streams video (large sporting events, awards shows, etc.).