If you use a lower bitrate/resolution/FPS you might be able to do it. But it'd be worse than ggyg's stream so I wouldn't suggest it. Capture card or upgrade.
EDIT: I can do 480p @ 24 FPS 2000 bitrate without any noticeable lag/effects. My FPS counter in TF2 stays constant regardless but it definitely spikes because of the CPU strain on encoding. Phenom II x3 3.4GHz, 4GB DDR3 1333 MHz ram, and a 4850 radeon. Any higher resolution and I get spikes of lag (especially when loading new areas).