Is firewire better than USB for live webcasting - Video
For the best quality and/or High Definition you want to use the fastest computer input you have that your streaming app will accept. That would normally be Firewire. Most online streaming sites will accept both USB or Firewire.

Take a look at http://www.youtube.com/user/TEDxTalks/search?query=tedxsanantonio+2011. These are TEDx Talks we shot using a SONY PMW-EX1 with HD/SDI out and a few other SONY HDV cameras with SDI adapters ($200 each I think) into a Newtek Tricaster that was streaming and recording 1080i. The HD files were uploaded to YouTube.

Hope that helps