Quote from WinstonTJ:
Dual WAN is the way to go since the router will keep the same IP assigned to the computer regardless of which line is actually feeding data. If you switch networks on W7 (possibly server 2k8 but never used it) its a royal PITA, you need to attach monitor + keyboard and select "work network" and add the new MAC address of the new router if running dual routers.
If you go the dual router route and choose to run both into the same PC then I'd suggest using the same IP/Gateway for both routers and just assign the server a different IP address (say 192.168.1.10 & 192.168.1.11) where each router has both IPs and same gateway, DNS, subnet, etc. so that if one line goes down the system can easily adapt to the new primary feed and if you ever unplug the cables it doesn't matter which port you plug them back into.
Also make sure you swap both intentionally so that both lines work on both NIC ports. Also need to set up remote desktop/VNC/general permissions & access on BOTH ports for BOTH lines so that you have full redundancy. If not you may have data but no access to your server.