That would be nice if there were a monitor that could support it.
Well, in case someone missed the point there -- it's not about displaying. It's about using the rather absurd number of specialized transistors on the GPU to do film-grade rendering in less time, and then dump it back to the HD, which is possible to do now with PCI Express. That would lead to another significant drop in CGI costs (and even more commercials using CGI ever so badly).
You still wouldn't want to use the entire 512M as a frame buffer though -- you really, really want to load textures and other stuff into the memory or else you'd be killed by transfer rates.