As far as I have seen from Apple, this will only work nowwith the new iMac. It's possible that future Macs would have this ability (even without the iSight since the Mac mini would make a nice digital media hub when connected to a plasma or LCD HDTV), but that's just speculation off of the top of my head.
As for older Macs, unless it's an aftermarket add-on from third parties, I don't see this working on older Macs. And even if these options were available, it would only be for the faster Macs (possibly those that have the latest G4s and all the G5 machines). And yes, the vidcards would have to support stuff like Core Image and other features to handle the effects and the like.