Ram upgrade Macbook 5,2 early 2009 detecting only half of each slot of memory

Hi everyone,

I have recently bought a pair of 4GB DDR2 PC-6400 sticks to upgrade my Macbook. They are 2x4GB DDR2 800MHz.

I have seen a lot of people telling this kind of memory (800Mhz) works with this model, although it is stated to work with 667MHz.

The point is: I have already installed them, they seem to work correctly in "About my Mac" excepting one little thing...Onlye 2GB each are recognized!

So I have no errors, my Macbook recognizes 2 slots being used at 800MHz, but with half of their capacity.

I have tried almost everything:

Plugging in just one stick (detects 2GB@800MHz). Mixing them with my old 667Mhz (detects 2GB from de 4GB one at 800MHz and 2GB from the 2GB one at 667MHz, overall frequency 667MHz. Everything works fine if I put again my olds 2x2GB @667MHz, so it has nothing to do with the slot.

Resetting NVRAM, SMC

Memtests: al appear to be fine but half of the capacity (no errors stated).

Reinstalling a partition from scratch in Mountain Lion.

This is just the way it is? The 800MHz are like "cutting" the RAM capacity in half?

Thanks in advance