If you plan to buy additional GPUs for mining, I would recommend 5870s rather than 5970s as these have a higher standard clock and more headroom for overclocking (assuming you have the motherboard slots for twice as many GPUs)
I believe this is false, the 5970 can clock just as high as 2 crossfired 5870s. It can also do this using less power.
The GPU chips in a 5970 are the same as those in a 5870 and will use the same power at a given clock speed and voltage. In addition, a 5970 has an extra PCI bridge chip in it which uses some power, though this is probably not significant.
The most significant thing is heat dissipation - the heatsink and fan in the 5970 has to get rid of the heat from 2
GPU chips and is constrained to be the same size as that in a 5870 which only has to get rid of the heat generated by 1.
According to Wikipedia, http://en.wikipedia.org/wiki/Comparison_of_AMD_graphics_processing_units
The standard clock for 5970s is 725MHz, at which speed the card uses 294W of power.
Standard clock for a 5870 is 850MHz, at which speed each card uses 188W of power, 376W for 2 cards.
In terms of work done, a 5970 offers 725*3200/294=7891 shader.Mhz/Watt and 2 5870s offer 2*1600*850/376=7234 shader.Mhz/Watt. The 5970 is more power efficient at standard clock speeds because it is run at a lower voltage - the standard voltage of the GPUs in a 5970 is 1.05V and for 5870s is 1.15V. To overclock the GPUs in a 5970 to the same level as would be achieved with a 5870 would require the voltage to be set to the the same level - not impossible, but the power use would then be the same.
I overclock my 5870 to 980MHz at standard voltage, at which frequency it uses about 215W of power. If you do this on a 5970 it would use about 430W - not impossible, but cooling it would become a major issue.