also consider the power cost of the cards, heat generated and fan noise. the 5830 supposedly can pull 200+ watts each the 6870 cards can easily hit 280Mh without underclocking mem, cost about 180 bucks and use about 160w (with stock memory/core clocks). if you space them apart a bit they only run at 70º when overclocked to 975 and the fans at 60%
if you only have a few cards in a machine it wont make much of a difference but if you have 4+ cards on a motherboard it starts to add up (more wattage every day, more expensive power supplies, more heat, more fan noise/ more power to the fans, etc)
Please show me where you got that data from.