Trend the router's CPU usage for a week -- that's the best way to tell if you're approaching a device's throughput limit.

Have a look at http://www.cisco.com/en/US/products/sw/iosswrel/ps5207/products_feature_guide09186a00801d1e95.html.

3DES performance is terrible on Cisco routers when it's done via software. For example, a 2621 maxes out at ~110KB/sec.

It's hard to imagine an unused interface's resource usage being significant.Usually unused ports are shutdown for security reasons.

The switch will (silently) drop packets, and retransmission must be handled at a higher layer (e.g., TCP).