Both of the cards you've listed will really struggle to play any modern game, so if your upgrading because you want to play games then you've brought the wrong PC unfortunately.
However if your just interested in watching video's or blu-rays then the GT520 would be fine.
At the risk of sounding rude, it is just plain old and slow. Even my laptop is horrible at gaming and it has better specs than that computer. In order to play most games on the market at a lower resolution with the graphic settings at a moderate level, you need to spend as much on a video card as you spent on the whole computer.
The gt 520 will be a huge improvement over the onboard graphics of the motherboard. You will be able to play games like Diablo3 at lower graphic settings in low resolution. Forget about playing games like skyrim or bf3. If you can upgrade the powersupply to something with a bit more power then you can get a better graphics card but then you are going to run into the issue with your cpu really bottlenecking the system. Most newer games require a minimum dual core processor and some require a quad core. Your single core running at 3GHz even with hyperthreading is going to struggle on newer games.