7970 is a dual gpu video card, monster size, monster power, monster price but gets great performance.
7970 is NOT a dual GPU card. The 7990 (made by only a couple brands since it's not technically a real card pushed out by AMD) is the dual GPU card.
The 7970 is the flagship card for AMD and currently the best single video card solution (beating even the 680) but the 660ti is the 3rd highest on nVidia right now. nVidia's prices are always higher because there are so many people out there who swear that they're the better brand (not true, they're very even).
In my opinion (excuse my low post count, I'm new to aquariums but a veteran in computer parts), you should get the 7970, as it has more VRAM and a higher memory bus than the 660ti, which is definitely more futureproof than nVidia cards. You will have to take in consideration of having ~500w power supply at least and enough room in your case though.