I used to manage a Full Tolerance Infrastructure.

2 Devices of Everything (Routers, Firewalls, Edge Switches, Core Switches, Access Switches, etc)
2 Internet Providers
2 Private Circuits

Network Documentation is very very important, for Network Admins is a must, because for example a new team member joins, this information will be useful to acknowledge the network infrastructure.

I have experienced that T3 (DS3) collapses (saturates) once you reach 40 Mbps, even rate is up to 44 Mbps.