But will overclocking a 570 really improve it that much?
And won't I void the warranty?
Overclocking does help quite a bit, and generally it does not void your warranty. If you get a factory overclocked model, then you don't have to worry about the warranty. Overall, the GTX 570 is a good card, and you likely won't really feel that overclocking is necessary.
BTW, the 6990 and GTX 590 will not lower the prices of any cards, since they are not competing against anything but each other.