I get it now,thanks for explainingNo, not at all.
DDR RAM and GDDR RAM are both just RAM. The only reason GDDR is not used on mobo's is that it is not standardised and so you cannot just shove it in a slot unless it was made into a standard format and the mobo was made to accept it.
"normal" DDR RAM is standardised so that there is a basic set of rules that people can manufacture them to so they are interchangable with different mobo's.
If the price and performance was better with DDR than GDDR then gfx card manufacturers would most likely use that but again, seeing as price/performance is not favouring DDR at the moment there is no point spending money on two PCB designs to support two different memory types.
That is my understanding of it anyhow.