Merging clips

I am looking to make some humorous clips where I take some audio and merge it with other audio. For example, I'd like to take a few clips of different conversations and combined bits of them together to make a whole other conversation. Is there any other hardware/software I would need besides from my mac and garageband?

FFMPEGX maybe? Think there should be an option in there for what you want, check it out....http://homepage.mac.com/major4/