Are these pc parts compatible?

For that system you better consider a PSU at least 850W or better that 5870x2 needs 2, 8pin + 1 6pin PCIe connectors. A 300W wouldn't even start that rig! The RAM should be a DDR3-1600 triple channel kit. Hope you have a good case with plenty of fans, and are using at least a good after market CPU cooler, if not water cooler.