The Inq's sources hint that ATI's R520 will look like the previous Radeon X850XT PE card. So this means while NVIDIA's new GeForce 7800 GTX features a single slot design, ATI's upcoming R520 will have a dual-slot design.
The sources also claim the R520 will be clocked much higher than 600MHz. A definite clock speeds isn't set yet, it will depend on the yields.
The R520 (X900?) is slated for September. More info at The Inquirer