Techiton

AMD had earlier this month launched the new Radeon HD4890 which is based on RV790 chip design.
The HD 4890 is based on the 55 nanometer fabrication technology. It comes with 1 GigaBytes of memory, 256-bit memory interface and contains 800 stream processors. The HD 4890 card comes close to Nvidia's GTX275 card. More detailed specifications are listed below:

Update: Saturday, October 3, 2009
AMD has launched the HD5870 which beats HD4890 in terms of the fastest single-GPU graphics card.

HD4890 Specifications:
RAM Size: 1024MB GDDR5
Memory bus size: 256bit
GPU frequency: 850Mhz
Memory frequency: 975Mhz
Stream Processors: 800
TDP: 190W
Price: $259

Links to Review Sites:
OverclockersClub - Sapphire HD 4890 Review
X-Bit Labs - ATI Radeon HD 4890: 1GHz Conquered!
HotHardware - ATI Radeon HD 4890: The RV790 Unveiled
PC Perspective - AMD Radeon HD 4890 RV790 and GeForce GTX 275 Review
Bit-Tech.net - Radeon HD 4890 vs GeForce GTX 275
Guru3d - Radeon HD 4890 review | test
TomsHardware - ATI Radeon HD 4890: Playing To Win Or Played Again?
AnandTech - ATI Radeon HD 4890 vs. NVIDIA GeForce GTX 275