benchmark figures for AMD's new Radeon HD 4890 are out and it doesn't look like the card is a huge improvement over the Radeon HD 4870, at least on in the two Futuremark benchmarks that the card in question got tested with.
As far as the test system is concerned, we don't have the full details, but it's using a Core i7 920 processor and has 3GB of DDR3 memory and the operating system of choice was Windows Vista. In 3DMark06 the card scored 16,096 points which is roughly about 1,000 points faster than the Radeon HD 4870. The HDR/SM3.0 test was up by over 500 points as well, which falls in line with the more efficient Shaders that the 4890 is said to have.
In 3DMark Vantage the card scored P10996 with a GPU score of P9801. The test results are all at stock speeds, but don't seem to be very remarkable. However, newer drivers might still change things, as it's not clear what driver version was used and early beta drivers generally doesn't offer as good performance as production drivers. It's also worth keeping in mind that we don't know what the rest of the components of this system were, so it's hard to draw any final conclusion from these results.
You can find the screen shots and some more details here