I think both of these download rates (3.8 and 5.5 mb/s) should be sufficient for good streaming. Of course, that rate may fluctuate and may cause buffering occasionally.
I'm beginning to think that my video card is holding me back a bit. I purchased the DirecTV NFL Sunday Ticket streaming service. I can watch at MEDIUM quality and it works very well. But as soon as I try to go to HIGH, it's very jerky and never gets smooth. I would really love to be able to go to HIGH, because when I send to my big screen TV, the difference is significant.
My laptop has a good dual core processor and good memory and my download rate is very good. The only other thing I can think of is that damn video card.