Judging by that case's size and spacing ability (I've studied it thoroughly), I'd say that the maximum GPU upgrades that are available for you with that case are either Radeon HD 7770 or GeForce GTX 650. I personally recommend getting HD 7770, since it's a small card in general, while with some of GTX 650's models you could experience problems.
Basically, you can get HIS' "HD 7770 iCooler" or Sapphire's "HD 7770 Flex" and you're absolutely good to go for several years without changing much (or anything at all).
And you don't even have to sell/give away your GT 520, because later (when you get bigger case and a motherboard with more "PCI-e x16" slots) you can easily make an "HD 7770+GT 520" HPX setup, thus gaining extra benefit from your old hardware.
I bought the Radeon HD 7770 and its gonna be here tomorrow.
Share results and thoughts if you feel like to. Anyway, you're always welcome.
Enjoy your new card, it's quite of a good upgrade from your previous one, just remember that it's not for "Max Payne 3 All Maxed Out on 2560x1440 monitor"-like shenanigans. %)