NL's engine is still quite ancient by today's standards (ignoring the water and high poly trains/objects, which are fairly modern features but are very sparingly used), therefore it is designed to run on old graphics. Anyone can spit out a simple scene in NL and make it run at 60fps on the ancient Geforce 4 MX, but once you start adding supports, more trains, and trees, it starts dropping down. The average medium-heavy custom objects ride will sink down to 10fps on such a system.
The main benefit of having a modern graphics card on NL is antialiasing, water reflections, and ability to handle all that at high framerates even for rides with many custom objects. Despite the fact that numerous optimizations are done for shaders and other features that NL hardly uses, they are still able to push far more geometry than older cards, especially considering how simple almost all of the geometry is. (i.e. They're all single-textured and have nothing else; a handful of polys in a scene may have a very simple environment map if the modeler was creative enough to do that, but even those are very simple to render.)
I thought the whole 6900 series (or at least the 6950 and up) were basically dual core versions of lower end cards, but it looks like nVidia has very well caught up to ATI.