When compared on different makes and model boards the fsb, HT, DDR2 800, and a few others things are very much the same. The 590 is a slight upgrade as far as the chipset itself. One way to make a good comparison is to look over two different Asus models, one with the 570, one with the 590. A good look over of the specifications will see that other then the slight difference seen with the chipset the other features and capabilities are ??? close
The model with the 570 sees a lower cost and run 6 sata drives while the more expensive 590 model will run 8 sata drives. Both support SLI and will run 8gb of DDR2 800 memory. The main difference is a slightly newer and faster chipset while other things like memory, video and sound(optional) cards, supply, hard drive(s), and the OS will play the bigger role.
