Don't be too hard on yourself, we all make mistakes. They key things are (someone else chime in if I miss anything) is RAM type, as in DDR2, DDR3, etc. Speed, 1066 mhz,1333 mhz, etc. Size, 204 pins, 240 pins, etc. Voltage, 1.35v, 1.5v, etc. Capacity, (A plethora of gigabyte options per stick)...