motherboard - DDR3 memory modules: Should they be identical?

Is it true (or a legend) that when I insert more than one DDR3 1333MHz
(or, more generally, any DDR) memory modules into a computer, the modules should be identical and of the same model and vendor?

What about for example one 4GB module and another 2GB module into the same computer? May it lead to bugs?

Also: Should the modules occupy the lower numbered slots on the motherboard? (for example, if I use slots 2 and 3 of available slots 1, 2, 3, 4, may it lead to bugs?)

Answer

Yes, it is best practice to use identical memory modules. You cannot mix DDR with DDR2 or DDR2 with DDR3 or vice versa. You want to make sure each stick has the same cas latency, timings and voltage.

You can however mix different DDR3 models because they would have these same latency, timings and voltage. Where if one was slightly faster, the motherboard will underclock the faster one. This is still not recommended.

As for the where the memory modules should be occupied on the motherboard slots, usually motherboard specifies what slots should be used and organizing the slots or banks by color. It will not run in dual-channel mode if they are not in the correct banks with matching memory modules.