This is addressing comments about memory bandwidth. The hd4870 has 115gb/sec gddr5 memory, if you take the 4850's limited 64gb/sec memory bandwidth and apply it to the same clocked 800 shaders of the 4870 (750mhz ie an overclocked 4850) you only lose about 5% overall performance compared to the stock hd4870. The 5870 with 153+gb/sec gddr5 gives plenty of bandwidth for 1600 shaders considering 800 shaders at 64gb/sec gddr3 nets you 95% of 800 shaders at 115gb/sec gddr5. The bottleneck must begin somewhere below ~70gb/sec ddr3/5 for 800 shaders in ati's architecture. This would mean that 1600 shaders only needs greater than 140+gb/sec memory bandwidth to operate at it's fullest potential. I also found this graph on TPU where wizzard was disabling SIMD units in rv770 for his 4830 review to see how many FPS he could get with 80 to 800 shaders in increments of 80. Seems like as he enables shaders the fps increases linearly, which is good news for going from 800 to 1600.

http://www.techpowerup.com/articles/other/155