Some more specific details have emerged in regards to AMD's upcoming RV790 and RV740 GPUs, which give us a better idea of what to expect once they're ready for the big time.
Hardware-Infos has gained the full rundown of specs for each, but it should be noted that these chips are still in the development phase and any of the various specifications are subject to change. But knowing AMD, it's probable what we know of them now will likely stay in place.
Accordingly, the current RV790 samples work with 750 MHz on the chip and are equipped with 1024 MiB big, 900 MHz fast Qimonda GDDR5 memory, which is bound to a 256 Bit wide interface as unsual.
AMD's RV740 chips also do some testing rounds eagerly. Here samples with 700 MHz core frequency are going around - there are still experiments with the memory. So AMD bets, as suggested in face of the 128 Bit wide interface, on GDDR5 memory, too. This is between 512 and 1024 MiB wide and runs with 800 to 900 MHz.
You can read all the details here.