256mb of ram is very important if you want to run high end games with large textures at high resolution with anti aliasing and anisotropic filtering on. Games are going to be getting larger over time, and these huge, nice looking, textures are going to be getting more and more common. So if you want your card to last a while, get 256mb of video ram.
But of course, you must take into account, there's no point of having all that ram without a good GPU (6800, x800, etc) that can make use of it quickly enough.