AMD will probably do something similar to what they did on the old 780G type chipsets where manufacturers would solder some Ram to the motherboard for the GPU to use as a "cache" where it's memory pools in system memory and the motherboards memory could be accessed and used in tandem or individually depending on needs or user settings.
Besides, the greatest issue with APU's is generally memory bandwidth, 128bit of GDDR5 is still nothing to sneeze at and certainly far better than the 64bit of DDR3 that the 780G chipsets used.
Plus having 256bit memory interface would require more layers on the PCB, hence driving up costs and AMD *is* targeting a cost sensitive market with it's APU's, better to go with 64bit or 128bit and clock the memory chips higher.
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