Well the obvious thing to say is go as big as you can! The better spec you buy, the longer it will last you and the quicker the things you want to do will be.
I work with a lot of video, doing transcoding, editing and the occasional bit of motion graphics and tend to find my 15inch, 2.2ghz i7 quad with 8GB ram seems to do the trick.
Looks great when you use an external monitor too, so I'd suggest having that sort of setup and an external hard drive to store all the media you record/shoot/create!
But if you can go bigger with the better processor then I would if I was you!