The reason is twofold.
1) Few games use more than 2-3 cores, making half of the 6 cores irrelevant.
2) Intel sandy and ivy bridge cpu's have a more sophisticated architecture that produce 15-30% more work per clock cycle.
Here are some benchmarks that illustrate that. Even a cheap dual core G860 would be better. http://www.tomshardware.com/reviews/gaming-fx-pentium-a...
A G2120 will sell for about the same price, or a i3-3220 ivy bridge cpu is not much more.
Since a non "K" intel cpu will not be overclocked, any supported 1155 based motherboard will do.
The 7850 is a very good choice for a graphics card. It is the most important component of a gaming pc.
6gb would be ok, no game by itself will use more than 2-3gb. But to get dual channel operation, a 8gb kit of 2 x 4gb is normal today. And, intel cpu's are not sensitive to speed or timings. Any ddr3 1600 ram will be fine.