The E520 has AHCI "legacy" BIOS. be sure to get video card that matches that. I had one and loved it. See my tutorial Overclocking Dell BTX Computers. The E520 was the test case for that. Look at link in mt sig. also
Single slot cards fit, cards with wide fan setups can't be over 6 3/4" long. Dual slot single fan cards require modding.