Been a while since I used it. But if memory serves, ffmpegX has a very flawed AVI splitting. Files that are split can't be joined later. To be safe I'd normally split the AVI/MKV on Windows, because let's face it, those formats have barely rudimentary support on the Mac. To date there isn't a single program on the Mac that can do half of what VirtualDub is capable of. I'm speaking of framerate conversion, video filtering, AC3 5.1 muxing, etc. Not to mention precise keyframe splitting.