atulmaniya :
Gigabyte is cheapest of all and only 25Mhz Gpu clock behind HIS.
Its not all about clock rate. The HIS has more shaders(cores) than the Gigabyte. The HIS is one tier higher and better than the Gigabyte to put it simply.
Let me show you. The HIS is based on this :
http://www.amd.com/us/products/desktop/graphics/7000/7870/pages/radeon-7870.aspx#3
1000MHz Engine Clock
2GB GDDR5 Memory
1200MHz Memory Clock (4.8 Gbps GDDR5)
153.6GB/s memory bandwidth (maximum)
2.56 TFLOPS Single Precision compute power
GCN Architecture
20 Compute Units (
1280 Stream Processors)
80 Texture Units
While the Gigabyte, Sapphire and MSI are based on this :
http://www.amd.com/us/products/desktop/graphics/7000/7850/Pages/radeon-7850.aspx#3
860MHz Engine Clock
2GB GDDR5 Memory
1200MHz Memory Clock (4.8 Gbps GDDR5)
153.6GB/s memory bandwidth (maximum)
1.76 TFLOPS Single Precision compute power
GCN Architecture
16 Compute Units (
1024 Stream Processors)
64 Texture Units
Note the underlined and bold parts. More cores even if clocked slower can still be better than LESS cores at a faster clock.