To get a good SC2 stream, you first really need to investigate the 5 main components of what is required to stream:
1) Your network upload speed, as the upload speed is the hard limiter on the quality you can stream at
2) Your PC specs, as streaming is extremely CPU intensive
3) Your monitor resolution - If you want HD quality video, you need a HD 16 : 9 resolution
4) A website to stream to (eg, Justin.tv, Ustream, Livestream, etc)
5) Your streaming configuration
For #1, go to any generic speed test website and test your upload speed. If you are going for a good HD quality stream, you want your upload to be at least 2.5-3Mbps. Your actual stream bitrate should be approx. 65% of your max upload speed. So if you have a 4000kbps max upload speed, set your stream bitrate to be 2600kbps max. This allows you ample room for game network traffic as well as preserving your latency/ping times.
For #2, any of the new i7 systems will be able to support a very high quality HD stream. The newest AMD CPUs can do this as well. The prior family of Intel CPUs, such as the Q9650, can also do HD streams. Any CPU below the Q9650 and you are going to have to tailor your stream to match the CPU.
For #3, you are going to want a 16 : 9 resolution monitor as that is what HD video is actually at. This means 1920x1080, or 1280x720. You could do a different resolution, but then you are going to have to "convert" that size to be a 1080/720 resolution. ANYTIME YOU CONVERT YOU WILL LESSEN THE QUALITY!! So as a rule of thumb, ONLY capture at a 16 : 9 resolution. I personally do 1920x1080 for my SC2 (and WoW) and it is awesome. If you must convert your stream output size, then do this: Set your monitor to 1920x1080, and convert your stream output to 1280x720. Those 2 sizes are exact mathematical ratios so the quality loss will be as minimal as possible.
For #4, I highly suggest you stream to Justin.tv for 1 simple reason: Quality. Unlike Livestream/Ustream, JTV does not limit you on your quality. You can literally stream a Bluray 1080p quality stream there completely free. To see an example of the quality you can get, see one of my streams (I stream at 5,000kbps 1080p) here:
http://www.justin.tv/multiboxing/videos. For reference, the videos seen there are Adobe FMLE 3.1 - 1920x1080 capture - H.264 - 5000kbps bitrate - 224kbps mp3 sound - 2 second keyframe - H.264 4.2 Encoding Profile. The hardware required for a 5000kbps stream is Intel 980X CPU, so pretty high end, however, this is an extreme stream.
For #5, this is the magical part. I highly recommend you download the free Adobe FME 2.5 or 3.2. If you have an i7 CPU and high bandwidth, get 3.2. If you have lower bandwidth and/or a medium or lower end CPU, get verison 2.5. 2.5 is better for lower end streams, 3.2 for higher end. Also download VHScrCap, a desktop capture software, also free. From there you simply need to configure the 2 softwares to work together. There is a complete tutorial for how to set these things up, complete with videos and samples here:
http://multiboxing.com/forums/f2/exact-way-stream-fme-jtv-read-1116.html The minimum stream specs I would say for doing a HD stream are this: Set your monitor to 1920x1080 resolution, set FMLE to encode to 1280x720, use FME 2.5, VP6 encoding, set bitrate to 2000kbps.
Hopefully my gigantic wall of text post helps you and others out. Let me know if you have any question and I'll try and answer!