nVidia has given a response, indicating that they know about it and it is not really a bug. Well, it's not a bug in that they were willing to ship the device knowing about this design oddity. Their statement makes it seem as if accessing the last 512 MB introduced an additional overhead, so it was better to limit use to the former 3.5 GB as long as all the textures & such can fit into that allotment of memory. Continuing to ask more of the card (using higher resolutions &/or higher quality filters) to get a more detailed image will result in the card eventually making use of the 512 MB partition; at that point, the tradeoff (whatever tradeoff it is) would be worth it. So at least the memory isn't dead & unusable.