How much would a HD6850 , i5 760, a good P55 board and the RAM's that ares1214 mentioned cost? I think that would be the best option that you can get i5 760 is better processor than the 965 and the 955.
Please mention the monitor that you have now.
The 760 is only better if he is big on overclocking. Otherwise, due mainly to the higher clock speed, the 955 is plenty good, and more importantly a lot cheaper. i5 760+mobo would likely be around $100 more.