MotherBoards (MoBos)

I was currently reading about motherboards that can handle DDR2 or DDR3 memory, but not both. The motherboard can only handle one type, with this being said I was curious about why a MoBos that suported DDR2 memory could handle a Grphics card with DDR3 memory. I recently found out that the graphics card actually suports its self like a self contained mini computer. This will allow for buying cheaper memory and still go for a faster graphics card.