You can convert most RealMedia files to MPEG4 or H.264 using ffmpegX (see sig for link). You MUST use an mencoder codec, though. For example, "Xvid [AVI] (mencoder)" will work, but "Xvid [AVI] (ffmpeg)" will NOT.
You could also take a nasty brute-force approach and use Snapz Pro to capture the video as you play it. That would be a last resort, though.