You can now find a 1Ghz Sapphire Dual-X 7970 for
$388 with Sleeping Dogs and the Asus DCUII goes for
$518 on Newegg. Alternatively, the Gigabyte Windforce 3x 7970 is
$428. Obviously from price/performance, HD7970 wins right away.
But I think you should revise your decision between HD7970 and GTX670 Asus DCUII for
$380. GTX680 is terrible value against that Asus 670 card since it costs $128 more for 7-8% more performance than the Asus DCUII 670.
Can you please list your system specifications, monitor resolution and the types of games you like to play? Do you care for PhysX? Do you care for MSAA? Do you care for texture mods? That would help to see if GTX680 is worth nearly $100 more over the 7970.
If you want to go all out and spend $520+ on a high-end single-GPU despite this being the tail end of a generation, might as well wait a couple weeks as Asus should launch the Matrix 7970 and that card could very well overclock to 1250mhz.