New rumors suggest that AMD's Kaveri APU will not support GDDR5 memory. Previous leaks claimed that the chip would support both DDR3 and GDDR5 memory, hinting that the new integrated graphics would be powerful enough to warranty more than a 128-bit memory bus. This doesn't appear to be the case though, the new leaks suggest Kaveri will be restricted to DDR3 and will thus be limited to a 128-bit memory bus for the graphics part.
That said AMD have significantly redesigned the memory controller to get the most out of the memory limitations and even though the new GPU will be powerful, it will not be powerful enough to warrant more than a 128-bit bus. Early reports suggested that the “best” AMD Kaveri APU part would be roughly equivalent to the recently released AMD Radeon HD 7730 graphics card. It is also worth noting that having to add in GDDR5 memory would do nothing for AMD except add in extra cost to the production process and they’d have to find a way of incorporating that GDDR5 into the APU platform – no easy challenge.