You have a budget of up to $8000?
With that kind of cash i would start a whole system re-haul before worrying about the GPU. You are running an old dual core. I suspect any graphics card you put in will be bottlenecked by the CPU. Drop in to the Systems forum and look at the sticky at the top. Make a new thread using that template, and no doubt they will find the best way to spend that cash (which is way too much for a gaming rig BTW).
But for a single monitor, a GTX670 is the card to get. If multiple monitors (gaming across them, not just an extended desktop) is a consideration, a HD7970. Gigabyte, Sapphire, ASUS, MSI are all good manufacturers to pick from.
Though if that budget is a typo, you would do better investing the cash into a better CPU. At the very least it will allow you to upgrade the card without the fear of bottle-necking.