For conventional stuff the influence is pretty much zero. Any acceleration is dependent on plug-ins using OpenGL or CUDA and there are a million scnearios where that won't be the case. The only relevance for CS6 is the raytrace stuff, but that is only supported on specific cards as listed here. Both your cards are supported, but as many otehr users have reported, the gain in speed with higher-end cards isn't that great. The raytracer is just awfully slow. All other accelerated features are UI-centric like the fast preview stuff, so it doesn't really figure in the equation for final rendering. It just makes working on a project a bit faster (if it works). So in the end, your current card will be good enough and you can save the money. Unless you realyl max out everything, it's not worth the investment.
Thanks for your time...i really appreciate the response!
The other consideration is third party plugins which are GPU dependent. Magic Bullet Looks, Videocopilot's Element 3D and Optical Flares, Zaxwerks 3D Invigorator, Knoll Lens Flare etc, are all capable of using GPU OpenGL accelleration. But the bump in performance between the cards you've noted would probably not be huge, as both are reasonably high end.