The jump from 256 to 512 will not be substantial for most games on the market.
There are a few rare situations currently where this can be a plus... EverquestII with it's "Maximum" texture detail, for example, will exceed 256mb of vram. Users with very large/high-resolution monitor users may also see some improvements if playing with high-degrees of antialiasing (say like 2536x1900 with 4/6x AA in a game with heavy texturing).