ATI's partner just could not wait to state that they do have ATI Radeon HD4890 (RV790XT) boards available for purchase wherever you are. In order to properly reply to yet another disastrous PR call from the nVidia crew, ATI's partners are now saying they have boards.
If you call nVidia's partners, you'll get an answer that their products won't be ready before April 6th, shipping to stores anywhere between April 6th and 14th. All in all, instead of playing smart, waiting for the dust to settle and then release allegedly higher-performing part, nVidia is shooting themselves in the foot with a ever-unpopular term "paper launch".
Anyways, getting back to the subject, Radeon HD 4890 will come with RV790 GPU clocked at 850 MHz, a good 100 MHz jump over the original RV780 chip. Number of shaders stayed the same, while partners will rely on now troubled Qimonda or Samsung chips for overall capacity of either 1GB or 2GB, depeding are you using one or both sides of the PCB. 1GB GDDR5 memory features bandwidth of 124.8 GB/s which is the figure nVidia can't reach with their GeForce GTX 275 card (111.74 GB/s). ATI continues to feature fixed-function AntiAliasing unit, so expect very interesting results between these two once that you enable 8xAA or more.
If you're wondering why would nVidia releasing a weaker part (GTX 275 is slower and less capable than GTX 285) to counter Radeon HD 4890, that part will be faster than current middle-range product, GeForce GTX260-216.