Pardon the expression, but it sounds to me like you have decided to lose your hardware tinkering virginity http://support.dell.com/support/edocs/systems/dime520/en/SM_EN/parts.htm#wp1211782
This should help you, as it will give detailed instructions to help you along.
You will not have any problems playing games like World of Warcraft.
If you are adding a all you will need is the cable and a screw driver.
If you need a hand, let me know.