Well my advice was going to be just run the single 520 for now and upgrade to a better single card later (try to sell the 520 for a little more cash), but looking at the motherboard it only supports Crossfire. This means you would need two AMD cards, but a 520 is an Nvidia card. So, you can't run two of them even if you wanted to.
An AMD 6670 is around $75, and would be a nice upgrade for now. I'd probably suggest saving just a bit more and getting a card for around $100 like a 6770, which is a very capable card. Either card is way faster than a 520.