AlienWare x51 GPU Upgrade?

I have an alienware x51 with a 3770k and a gtx 640. It's a nice rig but I am not pleased with the gtx 640. I was wondering if i can cram in a 670 up in this bitch. I was also wondering if an AMD graphics card would work in this system. Thanks for any help. Smiley face. I'm 15.

An AMD GPU would work in it, however we will need to know the specs of your power supply, as well as wether you will actually have the physical room for it.