What is the best graphic card for e521?

When you decide on upgrading components for a computer, it's not just the graphics card you need to worry about. There is a term called "bottlenecking", where one or more components hold the rest of the computer back. It's the equivalent of putting a V8 engine in a Honda Civic. Even if you did upgrade the graphics card, the rest of your components are so old they probably can't keep up with it. For your computer the most powerful graphics card it can handle is the GT240, provided you don't upgrade the power supply. If you wanted a more powerful card, you would need a power supply larger than the 305 watts you have, in which case the upper limit would be defined by which power supply you decide to buy. In terms of bottlenecking, the GT240 will still be the bottleneck for your computer. A graphics card on par with your computer's capabilities would probably be around the GTX 9800 or GTX 8800 range. Any card more powerful than that would be a waste.