I've recently upgraded the majority of the components in my rig, i7-3930k, SSD's, etc., but have held off upgrading the GPU. Currently I have an ASUS HD5870, and while it runs most games pretty decently, would like to explore high end single GPU options for gaming. I've looked at the Radeon HD7970 GHz, GTX 680, GTX 670 and Radeon HD7970. Help please!
If you're wanting to upgrade your system and continue to go down the single GPU path, then I would suggest two of the four GPUs you've got listed there. The AMD Radeon HD 7970 GHz Edition, and the NVIDIA GeForce GTX 680. Both of those GPUs are the best-in-class for their respective single GPU categories.
I've been a huge NVIDIA fan for years, usually preferring them to AMD just for their compatibility with 120Hz screens. But, I've been so pleasantly surprised with the HD 7950 that I run now. I would suggest getting a decent brand such as MSI or ASUS with their aftermarket coolers, and get whichever one you want.
I leave this open as both choices are going to give you a huge increase in performance, and an even bigger smile on your face. You have the MSI R7970 Lightning Radeon HD 7970 with Newegg for $489.99 or the MSI N680GTX Lightning GeForce GTX 680 for $10 more at $499.99.