The HD 7870 will draw almost 15A on the +12V rail(s) and require 175W of power from the rail when it is maxed out. Looking at the specs of your Antec, it appears to have the capability in the wattage and amperage area. And it has the necessary 6 pin connectors. Try it and see how it performs. If you experience shut downs or re-boots gaming, it will be because the PSU can't maintain the voltage when the card is maxed out. But I have built several of those 550W Antecs with similar demanding cards and had no problem.