The only reason you need to convert movies is so they can be put on an iPod. For that you'll need ffmpegX (or Handbrake if you're going straight from DVD). If you just want them in iTunes, it will play any movie which QuickTime will.
That might've been good information to impart in, you know, the first post... since you're on MacRumors. In that case, can't help you much... I've only ever done H.264 video encoding on OS X.