similar performance between the ATI 5770 and Nvidia GTX260, save your money. I doubt you would even increase FPS with your current CPU and a 5830, itself is not a great leap from a 5770.
As far as overclocking goes, go to Tom's OC forum. Post your CPU and mobo and they can help you out. http://www.tomshardware.com/forum/forum-29.html