3570K uses 70W at full load, perhaps 90W when properly OC'd with that cooler. 7870 uses 130-140W, or 150-160W when overclocked. Technically even a 400 watt unit would be enough but since you will need two PCIe connectors for that GPU, 450W-500W or more is recommended.
Rosewill Capstone 450W
$64, Gold efficient, has 2x 6+2 pin connectors
if you want to be covered for Crossfire in the future, which that board supports, there's Corsair TX750 V2
for just $70 AR AP. But it's "just" Bronze efficient