I know Rambus needs to be installed in pairs, but do they have to be equal pairs; like 2x64MB or could you install 1x64MB and 1x256MB?

We need some more info.

Do you know what kind of motherboard or the type of CPU? Do you have a manual of the motherboard or computer if it is a name brand.

Newer motherboards like Pentium II and above do not have a pairs requirement. I would say if your motherboard only has three memory slots then its no. And if the answer is no they probably do not need to be the same size but without knowing the motherboard model I can not say for sure. In addition, and again it depends on the motherboard you will need to know the max size of memory stick and the speed.

RAMBUS in intel chipsets are dual channel. Yes, you need similar capacity and speeds for both memory modules you are going to install.

It's a 1500MHz P4 and uses PC800 RDRAM. It has 4 bays, two are filled with 64MB and the other two have CRIMMS in them. I have one stick of 256 and would like to use by swapping out one of the 64 MB with the 256 if i can.

No, you can not. Did you read my post.

you could damage the memory channels by mixing different speeds and/or capacity values, they need to be in pairs.

Now if you bought another 256 mb stick, you can put those 2 in the other channel!!

Yeah, i read your post. I was just responding to michaelk. Thanks for the info.