I am assuming that you are putting two graphics cards in your system because you want to build a gaming rig. Although the dual cores perform good, as far as FPS and such, they do not touch the FX chips.
If you are a multitasker who just wants top end gaming when you happen to play, then go with the dual core. Dual cores bring absolutely nothing to the table when it comes to gaming, a single core is just as effective as a dual core GHz for GHz.(This may change if games start to utilize the dual core technology) If you are interested in overclocking, then again the single core will provide a better solution to your desires.
Gaming=FX57
MultiTasking=4000 X2