Hmm you may be right, but it was my understanding that there is some boost when you add a 3rd/4th card. It may be little compared to adding a 2nd card (and of course you would never need 4 5970s lol). I may be wrong though.
There is a little boost (a couple FPS higher) but it's not enough to need another card. And even with the fastest CPU, the cards will bottleneck so you'll see that the FPS stays level for all resolutions. When you see the flat line for the 3x 480, that means that the CPU cannot go any faster and so you'll get the same rates.
In this case, 2 cards perform better than 3 cards.
Overall, all I have to say is that it depends on the game you're playing and other components in your system. Tin Man, if you've never built your own computer or have no idea what I'm talking about, save yourself the trouble and just go with an easy build.