VR-Zone

The Tahiti chip is rumoured to debut the GCN (Graphics Core Next) architecture, featuring 32 Compute Units with a total of 2048 shaders. TMU count could be 128. The clock speed is likely to be under 1 GHz, providing a theoretical compute potential of around 4 single precision GFlops. Performance for Tahiti XT / HD 7970 could be close to today's dual-GPUs flagships, and considerably faster than GTX 580 and HD 6970. 

While most of the above information should be considered as rumours, what we do know is that Tahiti features a 384-bit memory interface, tied to GDDR5 memory. This should lead to memory bandwidth around 250 GB/s. Based on the PCB shots of Tahiti, forum members have used Photoshop to estimate the die size. As we speculated previously, the die size is now reported to be similar to Cayman - around 380 mm². Considering Tahiti will be manufactured at TSMC's 28nm process, this puts the estimated transistor count at a whopping 4.5 billion transistors.

Here's another die size estimate from Beyond3D forum :

AMD's Radeon HD 7900 series has been heavily rumoured for a release during CES 2012. The release date is now being pegged at 10th January 2012, Tuesday, and also the day CES 2012 commences. HD 7800 series is expected to release in February, with the dual-Tahiti New Zealand / HD 7990 flagship in March. The rest of the HD 7000 series will be rounded up by Q2 2012.

Source: 3DCenter