Don't want to turn this thread into an ATI/nvidia war, but in the case of single card solutions, nvidia are currently the best for high end cards.

When it comes into the crossfire/SLi realm, ATI just have better drivers and are more advanced in their technologies. Nvidia's SLi just isn't as good. I am actually really excited about crossfireX since it allows you to use 2 different ATI cards together which is proving to be awesome in benchmarks (drivers are still in development).