ATI will use GDDR3

ATI's future R420 will support three kinds of RAM: DDR, DDR II and DDR III
ATI made things big in order to perennialize the architecture replacing the Radeon 9xxx.
Indeed, when the new chip will be released, the top-of-the-range will be really expensive, since the market of DDR II is just rising.
The DDR III RAM is likely to be adopted much later for very high-end cards. Logically, the DDR will be shipped on low-range products.
Beyond forecasts, it's very likely to be one more marketing argument for ATI, in their raging war against nVidia.