SourceWe were able to take a peek at AMD NDA information (aimed at engineers) that details the technical features of the Kaveri APU. According to this information, Kaveri features a GDDR5 memory interface consisting of four 32-bit memory channels. This is perfectly matching the width of a GDDR5 chip which is also 32-bit. However, the memory controller has to be set up in a way so that two 32-bit channels work in tandem, half-channel use is not supported. The total width is 128-bit, so the main advantage comes from higher clock speeds of GDDR5 memory. This is in addition to the 128-bit DDR3 interface that we already know from previous APUs. Usage of DDR3 and GDDR5 is mutually exclusive.
While the information we glanced at is clearly preliminary and could be changed till the release of the chip, the focus should be less expensive GDDR5 chips with moderate clock speeds - not the 6GHz hotness you find in high-end graphics cards like GeForce GTX Titan. Specifically the document lists 800 MHz QDR and 850MHz QDR (3200MHz and 3400MHz) clocks which would result with 51.2 GB/s and 54.4 GB/s of system memory bandwidth. Compared to current 25.6 GB/s with the DDR3-1600, this is quite the performance bump. The surprises don't end there - Kaveri will support DDR3 up to 1250 MHz DDR (2500MHz) - it specifically adds 2400MHz and 2500MHz modes over Trinity, which officially supported up to 2133MHz. Nevertheless GDDR5 would provide a tangible bandwidth improvement and might be the smarter choice given that DDR3 above 1866MHz starts to get prohibitively expensive.