Visa has recently published what their data center looks like on their site. Check out their video here.
Visa has two data centers operating interchangeably and mirroring each other. Visa processes more than 80 billion transactions a with up to 30,000 simultaneous transactions. According to Network Computing, inside the pods, there are 376 servers, 277 switches, 85 routers and 42 firewalls and 3,000 miles of cable.
15,000 banks, processors and other third parties connect to Visa's network. Visa's network runs many information services (business intelligence, report generation, and risk management, fraud monitoring and encryption).