the latency, cl timings do matter too
if for example the old one is cl 1l and the new is cl15, both will run at cl 15
also matters size, some modules with different sizes will not run in dual channel mode
some mainboards are stubborn and will not work with different ram modules, no matter what
the voltage of the modules, be sure that both are 1.5 or 1.3 volts, not mixtures there is a must
sometimes is simpler to buy a ram kit and sell the old ram module