Docs at http://support.dell.com/support/edocs/systems/dimE521/en/SM_EN/specs.htm#wp1052310

It has a x16 PCIE card so we have no real power supply upgrade easy to find so let's get with just enough video to play a dvd?

Links ONLY from this point on.

1. http://www.tomshardware.com/forum/272444-33-what-asus-en8400gs-silent

2. http://www.amazon.com/EN8400GS-HTP-512M-V2-GeForce/dp/B002NU5SQQ/ref=pd_cp_e_3 (you'll have to tinker to make your own card cover.

3. http://www.amazon.com/EN9400GT-DI-512M-GeForce-Express/dp/B001MYHOS4/ref=pd_cp_e_2 (get the mounting kit.)