Nothing more than I already had on my PC or my N900. Remember, it's transcoding the files on the fly, so it's making it useable for the N900 immediately. You shouldn't need to add any codecs.

I should add, here, that it also does an amazing job of getting DVD video onto the N900 immediately, even from the DVD drive. This is nice if you've got a library of discs ripped to ISO on your computer's HDD, as some people do.