I would get the corsair 750w power supply they have it around 100$ after rebate (120 regular), Mobo hmm if your gonna get a 4870x2 get a x38/x48 chipset base mobo so you can crossfire or a p45 if your not gonna crossfire it.
for the heat on the 4870x2 just buy a zalman vf1000, it'll make it into a 2 slot video card and it'll have better cooling