any modern CPU (like the phenom II x4) is enough today's games especially when teamed with a good video card. the video card still does all the heavy lifting for games so if you took the price difference between the phenom II x4 & FX 8150 ($80) and placed that into your video card budget you would go from the AMD 6850 to the AMD 7850.
frankly with the AMD 6850, it's doubtful you see any real world difference between the FX-8 & phenom II x4 since both CPUs are plenty fast to keep up with the card. With the AMD 7850, you would notice a difference in gaming compared to the AMD 6850 especially if you are gaming at 1080p.
Awesome dude, you were really helpful. Thanks!