How important is to have matching speed of RAM? [duplicate]

I have a 4GB DDR3 800Mhz recently I installed another 4GB. It turns out it's DDR3 667MHz. Does it really matter I should own both rams 800MHz?

Screenshot

Slot1 shows recent RAM installation. ALSO I don't know if this is 1600MHz (it shows 800x2 and 667x2 for other) or just 800MHz ram.


Usually your computer steps down to the common lowest speed. So if you add DDR3 memory at 667MHz and memory at 800MHz then all of your RAM will run at 667MHz.

This means that:

  • Memory access will be slower than before,
  • But you also have more memory.

In most cases more memory trumps qua performance.