According to several reports from NordicHardware and Fudzilla, AMD may soon fill that gap with an as-of-yet-unannounced Radeon HD 5830 graphics card. No information is available in the way of specs other than the fact that it will carry GDDR5 memory and is expected to perform similarly to the older Radeon HD 4890. Two possible price points have been mentioned as well: $199 to match the aforementioned 4890, or $239, which frankly doesn't sound very attractive as it falls a bit north of the mid-point between the Radeon HD 5770 and 5850.
Fudzilla claims AMD has set a February 5 launch date. In addition, the company is reportedly readying a new and improved version of its "Cypress" Radeon HD 5870 graphics processor for the launch of Nvidia's GF100.