Upgrade GTX 260 to a ATI 6850 or GTX 460

Hi

I've recently built a new PC, 2500k, Asus P8P67, 4gb DDR3 1600 ram, but I'm still using my old Corsair 650w PSU, my old HDD and my old GTX 260. I have however, been considering upgrading the Graphics card, unfortunately with the Asus P8P67 motherbaord I have I can't run SLI, only CrossFire, so purchasing a 2nd GTX260 isn't an option.
My question is, is it worthwhile upgrading to a GTX 460 or a ATI 6850? Or should I save my money for the time being and consider upgrading a bit further down the line?
I mostly use my PC for gaming and obviously like to play the newest title with as much sparkle and effects as possible. My current monitor resolution is 1920 x 1200.
Any help would be appreciated.

Thanks
Jonny