Exactly how does Graphics memory work ?

Hi guys,
With the expected release of the 5770 and 5750 i have become curious as to exactly how the memory archetecture works on a graphics card.
By which i mean, is there a way to work out the pin count to each memory module ? do they differ or is it a set number. I recently re read an article about how GDDR 5 sends more signals per clock and im fine with that. Im more looking for the really technical stuff like how the system comunicates with the bus. This is mainly because i want to know physically what the restriction is with the 128 bit bus. Yes i get that its smaller but i am looking for more depth of info than that.
If someone could link me to a good article about this or take the time to try and explain it to me i would be very grateful.

Mactronix