OCWorkbench published a chart with a performance comparison of the GeForce 8800 GT, GeForce 9800 GTX, Radeon HD 4850 and Radeon HD 4870:
It is said that the HD 4850 is 20% faster than the 8800GT and HD 4870 is 20% faster than the 9800GTX.
As we all know, GeForce 9800GTX is only 10% faster than the 8800GT. We are able to get a clearer picture of the performance of these 4 cards as shown in the diagram below.
In 3DMARK06, Radeon HD 4870 will be 32% faster than 8800GT, 22% faster than 9800GTX and 12% faster than 4850.
AS for 4850, it will be 20% faster than 8800GT and 10% faster than the 9800GTX.