Chalk it up to ATI's confusing naming scheme. 6870 is a weaker card than 5870, also I think miners are not yet optimized for 6XXX.
A rig with two 5970 is very solid if you can find some. If you want to add to an existing machine, a 5870 is good.
more likely not "miners not optimised", but 69xx less suitable for GPGPU usage and bit more suitable for gaming thanks to improved ROP's and memory size/speed.A rig with two 5970 is very solid if you can find some. If you want to add to an existing machine, a 5870 is good.
google [questionably reasonable]difference between VLIW5 [58x0 GPU's]and VLIW4 [69xx GPU's].
