ATI's new flagship performance and high end graphics chips will have the ability to use GDDR5 memory. This is the next big thing that ATI plans to embrace.
It looks like GDDR4 didn't really hit off very well, as it doesn't bring a lot of advantages over GDDR3, but GDDR5 promises some crazy frequencies all the way up to 2.5GHz / 5GHz effectively, which is more than two times faster then what you can achieve today with GDDR4.
This means that the new memory will have more bandwidth and this is something that graphics cards always enjoy.
At this time, it's unkown if the Nvidia GT200 will have GDDR5 support.