Your 6400 has the Apple Video System option installed, so if you do an advanced search of the Knowledge Base for it (select "Include content created prior to 1997"), you should be able to find some info about it. As I recall, it shipped with the Avid Cinema program, but the Apple Video Player program functions with it also. There was also an option for a TV tuner or FM tuner, allowing you to watch TV or listen to radio on your 6400. The Apple Video System was more of a high-tech toy at the time, but 10 years later - it's really low-tech. It was certainly never intended for high quality digitizing of analog video for full-screen display. The 603e/180 or 200 MHz processor and limited 1 MB of DRAM for onboard video support aren't capable of handling serious video work. For amusement purposes, you can connect a VCR and watch small-screen video and/or create and save small QuickTime video files.
Incidentally, the maximum supported RAM in a 6400 is 136 MBs - 8 MBs soldered to the motherboard and a pair of 64 MB DIMMs in each of the two RAM slots.