20 can be smooth depending upon what youre used to. Of course if you go from a desktop and get over 70FPS on any game you throw at it, and then you go to a desktop and get between 20 and 30 then of course it will look relatively choppy, but playability doesn't hinge on something being super smooth. I forget what movies run at, I want to say 24fps or 32fps, I forget, but it's definately not terribly high. The point is that you don't need to have super framerates to give the illusion of motion using still pictures, because that's essentially what you're dealing with, still pictures.
Personally, I would rather have the graphics card cause the slowdown. This depends on the game or app of course, but if you've ever experienced something running slowly because of lack of ram or cpu power I think you'd agree that that is worse. Nothing like pressing fire in a game and having the game hang for a second while it looks around on the hard disk for the file containing the associated sound.
Of course, this all depends on the game and how much it relies on cpu and ram vs gpu.
20-something fps is playable, not terribly pretty, but definately playable as long as it doesnt terribly hinder reaction time in game by not keeping the new images coming fast enough.