Here are the two side by side based on specs,
Video Card Comparison - GPUReview.com
Despite having "better" numbers, you can see that the GT 520 has about 1/3 the performance of the HD 4850.
That site is a great help in researching a new video card as not only do the list pretty much every card available today, they have links to multiple review sites so you can see how the cards perform.
Another thing to consider is the cost of the card. When the HD 4850 came out it was around $200.00, where the GT 520 is around the $60.00 mark. All things considering, unless there is a large gap in the time that the cards were released (more than 2 or 3 generations apart), there is no way that a $60 card will out perform a $200 dollar card.