Get the 5970 or two 5850s. The 5970 performs equal to two 5850s and costs the same as two GTX470, but has the advantage of only using 1 PCIe slot since it's a single card.
Two 5850s will give you the same performance and will be $100 cheaper, but is two cards.
Two GTX470s will give you better performance than a 5970 or two 5850s, but will run hotter and use much more electricity.
You can run 2x 5850s or 1 5970 on a 650watt PSU, but you'll need a 750-800 watt PSU for two GTX470s.
So if you want the most raw power, get two GTX470s in SLi. If you want great performance but a cheaper option, two 5850s will do nicely. And a $700 5970 will be a great performer as well...if your case can fit the 13inch card.