There are some Firewire devices out there that are essentially video capture interfaces.

I have had good luck with both the DataVideo DAC-100 for composite/s-video inouts and the Canopus TwinPact-100boxes as well for this plus RGB + VGA inputs.

Link to DataVideo:
http://www.datavideo.us/products/dac_100_main_page.htm

Link to Canopus:
http://www.canopus.com/products/Twin...winPact100.php