^If you're going to go phenom get the 1090t. FX and Phenom 2 are both fine (Phenom 2 actually has better performance, most of the time)
As for crossfire, it could benefit but I would honestly sell your 6870 and use the money for a newer card (560 ti, 6950, 6970 etc.) If not, CF with a 6870 will be pretty good still, like you will play games at high-max settings, depending on the game.