http://buyersguide.macrumors.com/#iMac
Any particular reason it has to be an Imac? Just buy a Notebook and a HP External screen if it's only for Photos and some video. That way you have the portability option later on. The white Macbook just got updated with the new video card so if you buy that, you'll be set when the new operating system comes out which means you'll be good for a LONG time. It's 999 but with a student discount it'll be 950. If you don't buy it from the Apple store there are places online to get it even cheaper.
I make movies all the times on my new macbook...as long as she isn't making movies with special effects using Final Cut Pro you'll be ok. You can use Final Cut on the Macbook (I do) it just takes forever to render b/c of effects like Magic Bullets. If you're using iMovie, you will be fine.