For NVidia, going from an x50 to and x60, x70 or x80 will all be notable steps up. the x50 series really isn't designed to handle much outside of browser games and productivity apps. The x60 is entry level in the performance series. x70 is good but budget conscious, and the x80 is all-out performance. Within those, you have the Ti and some other enhanced models that trade various things around, but basically that holds. In my experience, an x70 of one generation will perform about like an x80 of the previous generation and so on down the line.

I have a GTX580 Ti that performs better in some 3D games than a plain-jane 950.