I've got a LOT of music but no videos. Right now, I would assume that I would use most of my space for music and apps. I'd never really considered videos but the thought does intrigue me. On average, how much space does a movie take up?
Depends entirely on the compression, movie length, and original file size.
You can get a 1080p movie to 4GB without any massive change in quality. 720p would be about 2-3GB, and then a DVD is generally around 1.1GB.