The built in composite video input will work well for video capture.  I've done so using software called BTV Pro which seems to have been discontinued.  It runs under OS 8-9 and is a great app.  The most important factor is HDD write speed.  I never had an internal SCSI drive that would write fast enough to not drop frames, so I installed a FireWire PCI card in my 8600 and used that as the scratch disk.  Once I did that, I had a nice little setup.  Another (probably better) setup is using an EyeTV 200 with FireWire to capture video.  The quality was about the same, but the compression was better and allowed for easier viewing.  It would run under OS X though.