I work on simulators, but they're gunnery simulators, not flight sims. Typically our systems might run 3 or 4 computers in a rack, each specializing in a particular task & communicating on a LAN. The OS
is usually commercial to keep costs down, older systems using DOS or an older Windows, sometimes Unix.
I was working on something in Trenton a few years ago & I took the opportunity to visit the guys running the CAE
Herc simulator there. They run a proprietary program on C+ I think, but the big difference is in the video processing power. There are two rows of racks that mount video cards. They aren't full of cards (upgraded?), but there's still a lot of computing going on in there. There is an isolated anti-static floor, & cooled air flows underneath it & up through the racks, cooling the video cards. There is another seperate room with the hydraulic pump for the motion system & a power conditioner which isolates the system from the commercial electrical grid & provides clean voltages.
Although our simulators get more complex with each generation, it was very interesting to compare them to the pinnacle of simulation technology.