According to VR-Zone, AMD will be launching the HD4890 sometime around mid April. Previous speculation stating this card would be dubbed the HD4970 was incorrect. The card will have 960 stream processors and 48 TMU's. The RV790XT sample has an 850Mhz clockspeed and should be paird with 975Mhz GDDR5 memory.
The card will be in two editions: standard and OC. The former should hit in mid April while the latter will come by the end of April. The Radeon HD 4890 OC will have 1GB of memory onboard and reports are saying that it is 25%-30% faster that the HD 4870. You can expect a $299USD launch price when it hits the shelves.
Meanwhile, TechPowerUp says that the RV740 will be released along with AMD's lineup for April. The card is expected to have 640 SP's, 32 TMU's, and a 128-bit wide GDDR5 memory interface. No word has been said about the price of the RV740.
Both of the new cards represent AMD's efforts to reduce the die size from 55nm to 40nm.