OutThere said:Final Cut Pro is a professional video editing app and typically doesn't like to play nice with various video codecs, as it's designed, and used by most, to edit with DV/HDV(starting in v5)/Uncompressed video, and typically is not used to import, say, a DivX video downloaded off of BitTorrent.
Now, since you have Azureus and Limewire in your dock, and, speaking honestly, neither of those programs have many legitimate uses, I'll assume that you're getting the video files through those programs, and possibly FCP. I don't have any problem with that, I'm just making an assumption so I can give you some more precise advice.
Now, if you don't really intend to learn and use Final Cut Pro, don't bother. It's easy enough to open up and play with the clips, but you will waste hours trying to do simple things that you could easily do in iMovie. If you get the DivX codec (http://www.divx.com/) you will be able to play DivX compressed movies in Quicktime, and therefore import them into iMovie. Final Cut Pro isn't always as nice, and usually the easiest way to go about getting video from different codecs into it is to convert it using Ffmpeg (http://homepage.mac.com/major4/) to DV or the like.
I'm a film student and I come from Premiere Pro, etc, so I'm very familiar with media and I don't try to import temporal codecs in FCP. FCP was actually one of the driving reasons to get a g5, A LOT of the editing market share is held by FCP, and to some degree Motion. I really like compressor from the experience I've had with it. Azureus/Limewire aren't there to get media to edit from.