M17x R2 Revisited

Hey everyone , I decided to go backward and picked up a max spec m17x R2. The reason was simple. I love the WUXGA screen , all aluminum top and bottom chassis, and it's a 17". My question is , What is the max GPU upgrade available , either nVidia or AMD that will work in this laptop.

Current spec has Intel 940XM - 8GB RAM - Dual AMD 5870

Thanks in advance.