RAM msi m670

Hello, I just bought a 2 GB memory DDR2 800mhz in a MSI m670 XP but does not recognize, I even tried starting my PC in Safe mode or normal black screen after I put on 1 or 2 slot would you counsel me on this problem, I was thinking my PC does not support 800mhz.
But later i realize that 2 GB ram recognizes in Bios is this because its memory 2438ghz.

Thanks in Advance.

I have the RAM with the 2 GB which met in 667Mhz.

What is the exact model of your PC? MSI m670 s1? MSI m670 S9?

Because if it is an old model, it does not may be strips of 2gb ram, as on my laptop. Either a BIOS update can fix it, which is rare, or you have to buy 2 * 1gb up!