The site claims the RV870 will offer a computing power of 1.9 teraFLOPS, compared to 1.2 teraFLOPS offered by the Radeon HD 4870 (RV770). There will also be a dual-GPU version of the RV870, with higher clockspeeds that model should be able to touch the 4 teraFLOPS mark.
Unless ATI changes it's naming scheme, these chips will likely be named Radeon HD 5850, Radeon HD 5870 and Radeon HD 5870 X2.