3 480's is not better than 2 5970s in terms of raw power, however 4 480's is, if your going for the best possible setup 4 480's is the best right now.
Also 400 series > 5000 series. 400 series is newer and is better for DX11 then the 5000 series, the 400 series has good tesselation and with 3D Vision and Physx I don't think you can go wrong.
With 4 480's you're going to need two need the largest power supplies and you will not be able to cool your computer naturally.
What cooling system would you recommend? I have seen a few people running 4 480s in Sli at load at about 80-90c naturally. (Fan)