Higher bitrates are going to help your image quality, but if you're not a partnered streamer, will increase the amount of 'buffering' your viewers will tend to receive, increase your delay, and in some cases aren't needed.
5000kbps+ - 1080p@60fps. Do not use, for a wide variety of reasons it doesn't work well.
3500kbps - ABSOLUTE MAX recommended by Twitch. Higher than this will cause the ingest servers to start having problems. Good for 1080p@30fps, or extreme quality 720p@60fps.
3000kbps - 1080@30 at acceptable quality, 720@30 at good quality
2500kbps - 720@60 at acceptable quality, 720@30 at excellent quality
2000kbps - 720@60 at very poor quality,720@30 at good quality
1500kbps - 720@30 at acceptable quality, 540@30 at good quality
Less - not tested, but 1000kbps is about as low as I'd go, possibly a 480p stream.
The recommended maximum for NON-PARTNERED streamers is 2000kbps. 1500 is sometimes required, if buffering is still significant. 3500 will give you excellent visual quality, but will likely be an unwatchable buffer-fest for most of your viewers unless you are partnered and get access to transcodes, the alternate CDNs, higher prioritization, and so on.