Speed doesn't really matter. Motherboards, at least anythgin made within the past 4 years, automatically scales to the speed the motherboards can support. As an example i've put DDR400 RAM modules in my other PC mixed with lower DDR modules, it scales to the lower module's speed, e.g PC2100 vs PC2700, the motherboard will AUTOMATICALLy set it to the lowest standard piece you have.
Basically to sum it up...it goes to the highest speed the motherboard allows or the module allows. It auto-scales it.