The speeds don't matter. It *should* slow down to match the lowest clock speed.
The next main thing to check is if it's actually in properly. I remember the first time i installed ram i pushed it in and found the clips wouldn't close, i tried starting up my pc but it still showed my old amount of ram. So i took out BOTH sticks and compared them, they were exactly the same. I was suprised at the amount of force you actually need to get it in. Now i've learnt that you need to give it a good push in till you can feel the scrape of the connectors in the socket, at which point you can close the clips.

I was too gentle with them before, also check to see if it's the right type of memory. Your motherboard could support 2 types, 1 bay of 2 slots can be 1 type and another bay can be another type. So check to see if your inserting the ram into the right slot or if it is the right type of ram anyway.

Hope it helps.
-Hellfire