which is best gt620 or gt520 for upcomig games in 2014 and2015

Solution
Neither. Save some more money, buy something for $150-$200. Trust me, it's absolutely worth it. You can always find some decent cards on sale.

Try for something like a 750ti. It should work great in any computer and give decent gameplay.

I mean odds are a gt620 (the stronger one) won't provide playable framerates in upcoming games.
Neither. Save some more money, buy something for $150-$200. Trust me, it's absolutely worth it. You can always find some decent cards on sale.

Try for something like a 750ti. It should work great in any computer and give decent gameplay.

I mean odds are a gt620 (the stronger one) won't provide playable framerates in upcoming games.
 
Solution
7750 will be your cheapest choice for gaming,this card is good for medium settings(maybe a tweaked medium settings) on a 1080p resolution.Getting those cards will be not worth it.

May I know your other specs? because I think you have an old system base on the gpu your asking to get,and getting a 750ti or stronger cards may just bottleneck your system.