You also don't have to stream at 720p. You can stream SD as well, that would further reduce CPU requirements.

Edit: I did this with elgato hd60 and OBS at a 2500kbps bitrate. http://www.twitch.tv/pseudopad/v/4574285

Using OBS gives you a more stable stream than elgato's own software, but uses a lot more cpu resources. I can stream at a higher bitrate with the elgato software, but it'll glitch out here and there.