As we reported before here, the R600XT or Radeon HD 2900XT will have two clock speeds. In 2D the GPU works at 507 MHz and the memory at 1028 MHz, while in 3D the GPU works at 757 MHz and the memory hits 1656 MHz.
The trouble is that in 3D the card is noisier than the Geforce 8800 GTX. The Radeon HD 2900XT cooler has only two heat pipes and it is not as massive as the G80 GTX ones and this probably makes the fan spin at higher speed and thus making the unnecessary noise.
Performance wise it should be able to match the 8800 GTS, but not the GTX or Ultra.