The pin count really matters when you're thinking of what motherboard to get for your setup. Certain ones support 200-pin memory, others the 240-pin memory. You'll want a mobo that supports 240-pin memory, as they are the newer and better-performing kind.
what's the difference between memory with 200 pins and 240 pins
40 pins.
Motherboard choice will dictate memory choice. Must be compatible. And even then, not all 240 pin memory is the same. That little notch in the bottom of the RAM module will also prevent you from trying to use the wrong memory with a particular motherboard.