Any gfx card that fits the PCIe x6 slot is compatible. But to keep from wasting money on a card that the CPU will bottleneck, I'd not go any higher than the GTX 960 or R9-380. Of course, your 500W PSU will limit you to a card it can safely support. Can you find the make and model of that PSU? Not all 500W PSUs are created equal. For instance, the GTX 960 will run on a high quality 500W with 2 x 6-pin connectors. Some cheap 500W PSUs won't have that. The R9-380 requires a 550W min.