An AMD Specification slide which details the upcoming Radeon HD7800 Series GPU's - HD7870 Ghz Edition (Pitcairn XT) and HD7850 (Pitcairn Pro) has been leaked. The HD7800 Series GPU's would feature the 28 nm GCN Core Architecture, Specifications posted below:

The Radeon HD7870 would be a Ghz Edition GPU which means it would be stock clocked at 1000Mhz. The GPU core would consist of 2.8Billion Transistors, 1280 Stream Processors, 80 Texture Units, 32 ROP's. The card would include a 2GB GDDR5 memory clocked at 1200Mhz equaling around 4.8Gbps Data Rate and a bandwidth of 153.6Gbps. Total Compute performance is 2.56 TFlops, TDP would be around 175W and less than 3W while operating at ZeroCore Power mode.

AMD Radeon HD 7870

1280 Stream Processors

Code Name: Pitcairn XT

2GB GDDR5 Memory Buffer

Release Date: March 2012

TDP: 175W

Price: 299 $

The HD7850 wont be a Ghz Edition GPU, Featuring 1024 Stream Processors clocked at 860Mhz, 64 Texture Units, 32 ROP's, A Total compute performance of 1.76 TFlops. Memory would consist of a 2GB GDDR5 buffer operating at the same 1200Mhz - 153.6Gbps bandwidth. TDP of the GPU would be 130W and 3W when ZeroCore kicks in.

AMD Radeon HD 7850

1024 Stream Processors

Code Name: Pitcairn Pro

2GB GDDR5 Frame Buffer

Release Date: February 2012

TDP: 130W

Price: 249 $

There's also said to be a 1GB Version of the card which would launch in March:

AMD Radeon HD 7850

1024 Stream Processors

Code Name: Pitcairn Pro

1GB GDDR5 Frame Buffer

Release Date: February 2012

Price: 219 $

The Radeon HD7800 Series will be launching within this month around 15th March, Launch of the flagship HD7990 is also expected later in March as revealed in MSI's leaked slide here.