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.