It does help with Final Cut X. I use both a 23-inch Cinema Display and a 27-inch LED Cinema Display, and it felt like the 5870 was really lagging having to drive both displays and handle rendering, complex timelines, and the like.
With the 7950, everything feels much much smoother; with both displays running, rendering/exporting times have greatly improved, and there’s a lot less (mostly zero) lag when dealing with large/complex timelines. I recently cut a music video shot with the RED Scarlett camera, and edited using the RAW 4K media and the whole process was smooth as silk. I’m sure having 42GB of RAM helped a great deal, but that’s my experience.