Well, it depends, you can definitely have servers with NVidia hardware, and you don't need to have all of them with it. (Amazon offers GPGPU servers on EC2 for example)
Amazon only offers very costly Tesla and Quadro cards on their GPGPUs. Super expensive. For scale, if you do not need the extra memory on the Tesla cards, what you want is commodity NVIDIA GeForce cards that are a fraction of the cost.