Take it with a grain of salt, but according to the specifications on the site the Radeon HD 4890 will have a 850MHz GPU and 1GB GDDR5 memory clocked at 3900MHz (975MHz x4). For comparison, the Radeon HD 4870 has a 750MHz core and 3600MHz GDDR5.
According to Salland, the Sapphire Radeon HD 4890 will be available by early April.