Sorry for the delay in replying - my email notifications don't work any more. The 6850 is nowhere near as good as the GTX660 - I would absolutely 100% recommend the GTX660 as your best choice. If you do choose a Radeon, avoid Sapphire. I've had three die on me now, each one died just after warranty expired.
EDIT: Or the GTX650 Ti if you want to spend less. It's still faster than the 6850, though the GTX660 is easily 50% faster still than the GTX650 Ti - it's a huge difference.
Look at my idea , I will upgrade my computer :
PSU: 600 W
CPU: i3 (I will upgrade it at may)
GPU: 660 GTX 2GB
RAM: kingston HyperX 8 GB
I think that will be a great upgrade , and a great gaming computer , right ?!