Putting out a stream mostly puts pressure on the CPU (and your internet upload of course). Getting a seperate device to deal with the streaming is common and recommended, I think.
Like the Avermedia Live Gamer HD, for example. But there's a ton of alternatives.
Note: A capture card doesn't fully take the load off your PC, though. So a lot of people have dedicated PC's to deal with the stream. Which is even more recommended, but not necessary.