Back when they were popular, flight sims needed some pretty hefty hardware to get them running. But I can't remember any of them ever needing 120 dedicated graphics cards just to get off the ground.
But the HD World does. A custom F-16 fighter simulator, it runs off 120 dual core PCs with 120 $US400 graphics cards inside them, all chained together.
All that processing power gets you 10,000 "entities" on screen at once, realistic explosion and destruction effects and "20-40 visual acuity", which is apparently as close to photo-realism as current projector technology can manage in a situation like this.
Oh, and it all comes wrapped in a 180-degree screen, along with a fully authentic replica of an F-16 cockpit.
If it didn't cost millions and millions of dollars, I'd already have one on order.