first make sure if you are using DDR2 or DDR1. Because you cannot swap DDR1 ram into DDR2 slot.
Then comes the speed, if one of your ram is using 400mhz try to get another ram of same speed but i would recommend checking your motherboard specifications and see the speeds supported by it of RAM. If it does support higher speed like 800mhz then you could get your RAM working on your motherboard and as for speed, the faster Ram will align itself to lower ram speed so you would be getting 400mhz speed from faster ram too loosing it 400mhz due to your other slower ram. And same thing applies for case latency.
About your question: check motherboard's memory compatibility;
try to get the same specs of ram you are using before.