I have an GA-970A-DS3P motherboard with an AMD FX8350 8 core CPU at 4.00 GHz, the chipset is AMD 970. Running Windows 7. The system has a 750w Gold PSU, 16mb DDR3 Ram at 1666 GHz. What is the best/most powerful GPU I can upgrade to? Currently have an old HD6670 and I cannot play most current games. I cannot currently afford a new system. Thank you for your help.