The E520 has a PCIe 16x slot. The problem is at the front the cooler cover which also holds the CPU in limits the with of the card there to one slot. At the rear there is only one expansion slot available. A 2 slot card will go away from the other slots in BTX. Fan location cam be an issue also. In general longt single slot cards work, short 2 slot cards can be made to work if you can cut bracket in 1/2, and cut vent in side of plastic cover to let air go out rear case vents. Long 2 slot cards are pretty much impossible. If hot air comes out of card towards the front a cheap expansion slot blower will clear the air out nicely. See some of my other posts for O/C advice on E520. You might be surprised.
I'm currently running PNY GTX750Ti...