Wow, that is such entirely old and weak spec I would advise waiting until you can replace MB, CPU, RAM, and GPU all at once. Reason being it's barely enough to run W10, let alone games. Replacing RAM, which is necessary, would be unwise until you can replace MB, because the new RAM standard has been DDR4 for some time now.
Even if you just replace the GPU, it will not perform anywhere near it's potential due to being severely bottlenecked by the CPU and RAM.
Another reason to wait is you will get better deals on everything, whereas if you replace just GPU now, by the time you can afford the rest, the price you paid for it would probably be enough to get a better one.
Sorry to break it to you, because I know you're probably anxious to get better performance, but that is the reality of the situation. I'm surprised that rig even has a 600w PSU, because it's way overkill for such spec. What brand and model is it though?