How "matched" do 30-pin SIMMS need to be? Is it enough that they be the same size and not less than the specified speed for the machine in question (but not necessarily the same speed), do they need to be the same size and speed (as well as the proper speed rating for the machine), or do they need to be identical chips from the same manufacturer?

I ask because I have three 30-pin 16MB SIMMS and, for obvious reasons, want to go looking for a fourth.