How to watch movies on a Mac without it being choppy?

I recently starting using Watch Instantly on my Mac (MacBook Pro, 2.16 GHz, 2 GB Ram, Leopard). Some of the times it works great but much of the time I get choppiness. I've closed all other running programs to minimize CPU load and tried Safari, Camino and Firefox as the browser. Nothing seems to help. It definitely seems like it's a CPU load thing because the status bar indicates that enough of the movies has buffered I think.

Any suggestions?