From this article at ArsTechnica, entitled “Coming soon: an external video card near you?”

“NVIDIA has just released a monster external graphics solution for people who have exceedingly high demands for pushing pixels. The Quadro Plex 1000 is a set of graphics processors enclosed in its own box, capable of rendering 80 billion pixels per second and powering multiple monitors with a combined resolution of up to 148 million pixels. It doesn’t come cheap: prices start at $17,500, and it is intended primarily as a networked rendering solution. It requires one or two free PCIe slots (yes, you can buy two and run them in SLI mode) to connect it to your computer.”