I went through and tried to do this a year ago. The answer to your question is of course entirely going to depend on what video you're compressing and what your expectations are.

I wrote down some of my thoughts on the subject from that time, here: http://synack.net/~bbraun/68kvideo.html Unfortunately, they're probably not in a very usable form for you, but the gist of it was I was unable to achieve a playback framerate I found acceptable at 320x240, which was about the minimum resolution I found acceptable, all on a Q630.

FWIW, the content I was encoding was random TV content from the 68k era.