plus your motherboard is basic model, very entry level. Xfiring with a cheaper motherboard like that will only giv you 100% (first GPU) and the second will in THEORY max at 25% of card one (PCIe lane one is x16, while the other card will only get x4, 25% of the bandwith the first card is getting. With that said, unless you want to sink more into a motherboard that does pcie 16x and 8x, or 8x and 8x (idk if there is 16x and 8x, or even better 16x and 16x, but if this even exists its probably hundreds of dollars). so for the price, one HD 7870 will work better, and wont work the power supply so hard like two GPU's would. 500w is eh