Can you chose the graphics card that you want to upgrade to? Without knowing it, a PSU recommendation isn't easy. Or, you could give us a set amount of wattage to work with for graphics.
For example, 50-100W, 100-150W, 150W-200W, 200-300W, or any other range for a graphics card that you think you'd buy. I recommend either the first or the second, depending on how much gaming performance you want. The third or higher than it is for high-end gaming and you don't seem like you'd want to spend several hundred dollars on a graphics card.
Also, would you mind if we ask what software you'll be using in particular? I'd like to make sure if it is GPGPU acceleration-compatible. If it is, then you might benefit from getting a specific graphics card family more than the others.