We already informed you that RV630, the chip behind the Radeon HD2600 XT works at 800 MHz core and if the card uses DDR4, the memory works at 1,400 MHz and can easily go up to 1,500 MHz.
The cards are coming in July, but they will be showcased at Computex in Taiwan, the show starts on the 5th of June. The reference design will enable 256 and 512 MB cards, but the biggest limiting factor is the 128 bit memory interface. Some cards will even use a 64 bit memory interface and this will be a pure performance suicide.
That is what the reference design claims and it will be interesting to compare this card with the 8600 GTS, but we believe that the GTS is likely to win this one.