Dan, Re: The big problem is usally the video card's bios, which will often map addresses (sometimes 512mb worth) starting at 3GB so as to stay out of the way of devices (like caching disk controllers) that go for addresses at 3.5 or (as in your example) 3.75GB.
That's a good point, but 64-bit Windows is not going to make this requirement go away. If you have only 4GB of memory installed, the video device will still grab a section of this for MMIO. The benefit will not be realized until one moves to >4GB of memory.