I dont think anyone refers to the gpu as 256-bit, or however many bit, it mostly refers to the memory bus. So far all high end cards have a 256-bit memory bus, although the x1800 cards have an internal 512-bit mem bus inside the gpu, but the mem is still on a 256-bit bus.
Moreover, I doubt we will see a full 512-bit bus anytime soon, because that will require a complex, expensive board layour, and modern gpu's are not starved for bandwidth anyways.
Also, there are more effective solutions to increasing mem performance through a more efficient memory controller (a la x1800) or an embedded memory chip in the gpu die (xbox 360)