I use a PCTV-Rave card, which was discontinued a few years back, but it still works perfectly for the kind of caps I do. I got 3rd party drivers for it which means I can use it with Virtual Dub to cap live shows direct to Divx.
I really have no idea about doing it on a mac though - but with it being designed for media stuff I reckon it wouldn't be too hard to get into.