So can anyone tell me why we are still stuck at 6.4GB/s in the pc market?
Why can't we have 1600MHz DDR and 256Bit PC memory and busses?

Bandwidth isn't everything. Newer DX9 games will incorporate more and more pixelshaders. This will shift the bottleneck from bandwith to pixel processing power, so this double pumped 800 MHz, effectively 1600 MHz 256 bit memory architecture may not give the performance increase in PS2.0 and PS 3.0 loaded DX9 games.