Just read this conclusion:
http://www.techpowerup.com/reviews/ASUS/GeForce_GTX_670_Direct_Cu_II/33.html
Here's a quote from the conclusion:
"Yes, you've seen correctly, the ASUS GeForce GTX 670 Direct CU II TOP is the first graphics card ever that I gave a perfect ten score."
I have the Asus GTX 680 DCII TOP and it's just awesome. The 670 version gets the same performance as a reference 680, but is even quieter.
Additionally theses Asus cards have high quality caps and voltage parts that other cards do not.
3 versus 2-slots:
The Asus DCUII 680 cards are 3-slots and the DCUII 670 cards are both 2-slots.
The "TOP" version has a slight higher overclock. While the rest of the card is physically IDENTICAL, the GPU's have been BINNED so the TOP versions have been tested to overclock slightly better.
So if there's little difference in price get the TOP version:
http://ca.asus.com/en/Graphics_Cards/NVIDIA_Series/GF_GTX_670
(The "TOP" version has a "T" and is named: GTX670-DC2T-2GD5 )