When installing or upgrading your memory, it is considered good practice to put the largest memory module in the Slot 1, and go in descending order from there. For example, if you have 3 sticks of RAM – a 512 MB, a 256 MB, and a 128 MB – the configuration should be as such:

Slot 1 - 512 MB
Slot 2 – 256 MB
Slot 3 – 128 MB

If you have two of the same size modules, it does not matter which one goes in which place, but they should still be side by side. For example:

Slot 1 – 512
Slot 2 – 512
Slot 3 – 128

The reason this is considered a good practice is that on older machines, the BIOS will go nuts if the modules are in a different order. It sometimes causes system instability, wrong memory readings, and systems that are unable to boot.