Como a GPU e a CPU compartilham a mesma RAM, a memória disponível na GPU será a quantidade de memória livre informada pelo SO.
Cliquei no botão ? ao lado de "Memória máxima de vídeo gráfico" no link que você postou.
Graphics Video Max Memory
The maximum amount of memory accessible to processor graphics. Processor graphics operates on the same physical memory as the CPU (subject to OS, driver, and other system limitations).
EDIT: (para sua pergunta adicional)
http://www.pcgamer.com/how-to-buy-a-graphics-card-six-things-you-must-know-about-gpus/
Memory: size doesn't matter, but bandwidth is critical
Selecting a graphics card because it has more memory is like choosing an exotic car based on the size of the gas tank. The unwarranted fixation on the amount of RAM is the biggest, most common mistake made by gamers looking to buy graphics hardware.