AMD's HD 5970 consists of two HD 5870 GPUs shoved under a single hood, but for one reason or another (heat, power consumption, etc), the memory and GPU cores come underclocked from their standalone versions. The GPU cores in a 5970 have been dialed down from 850MHz to 725MHz, while the memory drops from 1200MHz to 1000MHz.
That won't be the case with Asus's "Ares" videocard, which combines two true HD 5870 GPUs into a single package. Asus even says they plan to overclock the core and memory, though by how much is yet to be determined. You'll also find 4GB of GDDR5 crammed inside, twice as much as a standard 5970.
In fact, the HD 5870 Cypress cores are overclocked slightly past the default 850 MHz - though how much ASUS hasn't shown us yet. Also, the card features 4GB of GDDR5 frame buffer - 2GB per GPU!