I watched a Vdieo card comparison video of LinusTechTips and the GTX 570 (at an average of 58 FPS) and the HD 6970 (at an average of 64.2FPS) came close to each other and what I want to ask is:
You can try and find what games you would be playing and go from there. Can't really give you a money vs performance as I don't know what you're paying.