I am about to buy a new system, usually I just buy mobo etc and place the new components in my Chieftec Dragon I bought many many years ago. I currently have an 8800GTS and it is tight. The 5850 from my new build might fit if I could take out an HDD cage but I need the drives so for this build I decided I needed a new case.
I looked at every case I could find and made a list of sizes of the cases and video cards. I am sure someone will correct me if I am wrong but a 5970 is around 31cm. The Fractal design takes up to 29cm, so even if you can get it to fit it is going to be tight.
I don't have a 5970, I am buying a 5850 so the Fractal design was high on my list. However I may get a 5970 in the future so it seems to make sense to buy the biggest case you can get away with both in cost and available space or be prepared to change your case a few years down the line.
I want a good case and don't want to buy another for a few years so I am possibly splashing for either the Corsair or a Lian-Li full Tower, both of which should see me through as long as the Chieftech Dragon has.
I see there is a new Fractal Design in the pipeline and everything I have heard about the current one makes me want to buy one now and worry about larger cards later.
There is no clear answer unfortunately, you need to think about what you want in it, how many HDD's, GPU's and other cards you need to fit in and select one based on budget and what one suits you (that you like the look of).
Good luck.