You need whatever you need for what you do. |: Vague, but correct.
Oh, and HD video streaming. /me cracks knuckles
Well if it's real HD stuff, none of this high resolution low bitrate shite; you'd need at least 12/13 Mbp/s to handle it (ideally 20ish, hihi ADLS2+), and that would get an adequate quality 720 stream (that'd only be with 2.0/2.1 audio though).
Personally I don't count Youtube HD since videos are covered in artefacts from shitty bitrates, and I'm not taking into account that anything with lots of fast moving stuff needs a much higher bitrate to keep the picture from breaking up.
That's just working from averages, the less there is going on the lower the requirement. So one of those tacky love movies would be fine at 8ish, and something with lots of fast camera movements & shit flying all over the place would need a lot more.
Also keep in mind that if you want a perfect 1080 HD stream with no artefacts, optional subtitles & optional surround depending on the video you could easily be looking at up to 64 Mbp/s (without taking overheads into consideration). Pretty shocking really, though I doubt anyone's going to be retarded enough to set up a stream like that. 25 - 30 Mbp/s is more realistic & would only have a single Pro Logic II audio stream in it.
(why Pro Logic II? Because it plays nice with stereo systems & doesn't cause audio volume issues)
Luckily you can get near-perfect DVD quality video streamed down 2.5 Mbp/s connections with lovely Dolby goodness, though you'd ideally want 4 Mbp/s to cover overheads & blips etc.
/video
Yeah, I know a lot about that shit. They joys of steaming stuff around your house.
For gaming... 8 Mbp/s, a good ping & a good upload rate will cover anyone. Anything above that's just going to waste. Gotta emphasise the upload rate which tends to get neglected, it's pointless having a rapid down speed for gaming if you're only getting 0.25 Mbp/s up.