OP. Based on your other comments for your specs. I'd recommend selling your current pc and taking that money to build a new one with a ddr4 motherboard.

If you want to hit the max of your cpu, and are set on keeping what you have. 500 would probably be buying a 6600 xt and making sure there's enough room in your case for it. This might still have some bottleneck honestly. The problem is your PC is so old that you can't go out here and buy one of these $500 or so cards because 1 your cpu/gpu bottleneck and 2 your power supply will explode (joking, but probably shut off) most likely, unless it has some sleeper power supply for some reason.

You can find the wattage physically on the unit somewhere. If the watts aren't listed and its just one color. I would highly advise getting a good power supply that is way over what you need, so that when you go to upgrade it later, you don't have to worry about getting another power supply. 500W would be enough to power that machine most likely, I didn't look at each part on pcpartpicker to tell you. Take the wattage on that site and multiply it be 1.5, that will be a good indication. I'd say get a 750W or 850W and run it for many many years. Get a good one though from that PSU tier list spreadsheet.