Rumour has it that the AMD RV770XT will have a 512-bit memory interface while the slower RV770PRO will use a 256-bit one.
It looks that the current GDDR5 design sticks with 512-bit memory interface and that RV770PRO will use 256-bit GDDR3 memory. The RV770PRO will need a smaller cooler, as it is definitely going to end up the cheaper of two, and you can bet on a sub-$200 price.
RV770PRO will also end up with a slower clock than RV770XT, but we still don’t know what the current speed plan. It is sampled and it works, at least the RV770PRO with 256-bit GDDR3 does.