69** series video cards scale very well..and, depending on your setup, with those you can do some very serious damage! (thats a good thing) games will run well, fps will be high, and if your running multiple monitor setups your gonna be very happy.. however, sli on the GTX 5 ** series are also very good..and 2 gtx 570's will blow away two 6950's..
So, unless your looking to save money..I would sli over crossfire..Nvdia cards give you cuda, 3d vision, and great driver support