So you're definitely misunderstanding.
GDDR comes on the graphics card; DDR3 connects to the motherboard. You can't add GDDR to an existing system, but you can add DDR3.
In your case, you might want to add DDR3 RAM, but it will only improve the performance a small amount.
And before you ask, NO, it will be very difficult, if not impossible, if you want to upgrade the graphics card.