Czech Gamer published leaked benchmark results of the Radeon HD 5870. Take it with a grain of salt though, as these results come from AMD's marketing department. Four cards were benched in HAWX DX10.1 and 3DMark Vantage, including the NVIDIA GeForce GTX 285.
Besides the benchmark results there are also some comparisons:
Radeon HD 5870 is reportedly 5 percent to 155 percent faster than the GeForce GTX 285 and 25 percent slower to 95 percent faster than the GeForce GTX 295.
Radeon HD 5850 is said to be 25 percent slower to 120 percent faster than the GeForce GTX 285