Video input

I know many people use Macs for video-editing and such, so I wonder if any of you have any advice on how to input several live channels of RCA (or S-video) sources on a Mac.

I know you can use most tv-cards to get one channel, but how do I go about the problem to get multiple sources at once.

The best solution I can imagine would be some kind of RCA to Firewire converter that would just let me use a firewire hub to get several sources, but I have small hopes of finding such a converter.