Hi I want to ask the forum if anyone can bring some light to me regarding the 2013 Mac Pro GPU upgrade !? Can you upgrade the GPU ? I KNOW APPLE.COM offer you option to build up you Mac Pro with: Dual AMD FirePro D300 GPUs with 2GB Dual AMD FirePro D500 GPUs with 3GB Dual AMD FirePro D700 GPUs with 6GB But I want to know if I buy/build my Mac Pro with AMD D300 2GB can I upgrade later on with the AMD D700 6GB or I am stuck with what I buy/build the first time.!? Thank you Also I know the CPU, Memory, SSD are upgradable but I don't know nothing about the GPU !?