RAM stick/kit picking:
Mainboard RAM compatibility must be considered. Look at your mainboard manual.
If your RAM kit/stick is not to be found in the compatibility list, you might just go ahead and try, keep your fingers crossed. It might still work.
RAM speed should follow max speed, your mainboard allows, if the the RAM kit/stick is compatible but has higher speed.
Make sure your picked DDR3 RAM kit/stick for your DDR3 compatible mainboard. DDR4 will not work.
If you plan to mixed and match (DDR3 kit A with different DDR3 kit B), you can only rely on luck. Nobody can know if it will work or not.
This is the reason to pick the same exact kit/stick as the current available one or just simply replace the whole RAM kit/sticks.