HD 7870 ghz edition, can't go wrong with it! Best performance/watt out there (source: http://www.techpowerup.com/reviews/NVIDIA/GeForce_GTX_Titan/28.html)
Check out any HD7870 too! The AMD Radeon HD 7870 XT (tathi LE) will also do the job but i couldn't find anything available online right now.
There are some great deals on the GTX 660 Ti. I'd go for the MSI GTX 660 Ti PowerEdition OC for $280 with a free copy of Metro Last Light. It comes with an all-custom design and a high factory overclock. That will bump you up from mid-level to the high-end performance bracket. http://www.newegg.com/Product/Product.aspx?Item=N82E16814127696
http://www.techpowerup.com/reviews/MSI/GTX_660_Ti_Power_Edition/1.html
I have seen 7950's for under 280, but if you get a 7870, get the hawk edition from MSI. Don't have any issues with it and have a 30 percent overclock on it.