The Radeon HD 4850 will have a 625MHz GPU while the Radeon HD 4870 will be clocked at 750MHz. The site says the Radeon HD 4870 with GDDR5 memory will be about 30 percent faster than the Radeon HD 4850 with GDDR3 memory.
NVIDIA's GeForce GTX 200 series will be released on June 17th and the dual-GPU ATI R700 is expected in late August.