FUD Zilla claims the GDDR5 version of the ATI RV770XT GPU will have support for a 512-bit memory interface. The card will be released in June but you'll have to wait until July for volume availability.
We are getting back to the memory controller of R600, but possibly significantly improved, and this time the chip can cope with GDDR5 while the previous controller could only cope with GDDR3 and GDDR4.
As GDDR4 didn’t really get these cards any faster, ATI decided to go for GDDR5. We believe this is the future and this definitely makes the RV770 a bit bigger than the RV670 and this will be the most significant difference between the chips.