[Build Help] VGA upgrade

Im considering an VGA upgrade on my PC. Right now the configuration is:

• Intel Core2 Q6600 @ 3,2GHz (Noctua NH-U9)
• GIGABYTE P35-DS3
• 2x2GB Apacer DDR2 GOLD
• ATi HD3870 512MB @ 850/2500MHz
• Fortron BlueStorm II 500W
• AOC i2757Fm

I know the CPU would probably bottle neck a high end graphics card so I'm thinking about:

• HD 7850
• GTX 560Ti

What do you think?