iMovie importing formats and audio

Can anyone tell me what format I can input my videos into iMovie so that the video and audio can be separated? Or is this not possible in iMovie? Also what formats can I use to import into iMovie, I have only been successful with MPEG-4, .avi or MPEG-2 does not work.

Another quick question, I am currently using Snow Leopard with latest updates including iMovie '08. But I noticed that whenever I try to use transitions it crashes, this is also happening with subtitles. If anyone could hep me with this it would be great.