Depends on your UPLOAD speed shimmers, as well as how your CPU can handle both streaming & playing your game.
Putting at a higher bitrate will make people with potatonet (don't forget the Aussies) - unable to watch your stream.

To simplify here is a rough configuration : http://pastebin.com/rqmUF8nf wastedH

NP - let us know of your Twitch account and we'll check you out and further help you. <3

Aha ok. I just broadcast on a simple i7-3770k & 2 monitors.

A single PC is sufficient if your CPU can handle it (as I mentioned b4). 1 monitor is ok but you won't be able to read chat (however you can work around with using your phone/tablet) or see who recently followed you.
So minimum is 2 monitors. Ideally, the monitor setup is 3. Only reason is you'll start having a billion things open that you'll need to see while casting (notifications, chat, OBS, plugin etc etc). Also, you can have 2 PCs if you don't want to put too much pressure on the PC while streaming & gaming at the same time but you'll need a capture card (Elgato, Avermedia, Hauppauge). This setting gets slightly complicated but we'll cover that next time.