Which Graphics Card is better than the other?

Hi guys I'm new in this site and have a major confusion.So I want some help.I bought a pc with the configuration of -
1.Intel Extreme series DP67BG Motherboard
2.Intel Core i7 2600 3.4 Ghz with Turbo Boost to 3.8.Processor
3.Zion 4GB 1333MHz DDR3 Ram
4.1TB Seagate HDD
5.LG E2260V LED Full HD Monitor
I have a graphics card which is very ordinary.Now I want to buy a graphics card that can run latest games with their maximum settings in Full HD resolution.I research a lot and also get adviced from others but still I have confusions about the graphics card.The confusion is which one is better from other and I should go for the better one.The 2 graphics cards are Sapphire HD Radeon 6950 1GB & Sapphire HD Radeon 6870 1GB.Many of my friend told me to buy the HD Radeon 6950 but I'm not fully Convinced that 6950 is better than the 6870.So plz convince me which one is better than other that means Which graphics card I should buy that can help to run the maximum latest games with their maximum game settings in Full HD resolution.Plz help me and what should be the accurate PSU for the graphics card?