Just to clarify a few points:
1. The Time Capsule is a router, and thus does not need a router to work. It can work connected to a router and having DHCP turned off, but it's not necessary. It does, however, need to be connected to a modem to be able to share an Internet connection.
2. The USB port on the Time Capsule is to connect HDDs, printers, or a hub with HDDs and/or printers attached. It cannot be used to access the TC's HDD directly.
3. The only ways to connect to the Time Capsule to be able to access the hard drive are through Ethernet or Wi-Fi. As others have stated, Ethernet is best for the first backup and Wi-Fi is adequate for subsequent backups as they are usually small in size.
4. 802.11n (Wi-Fi N) can get theoretical speeds of ~170 Mbps on the 2.4 GHz band and ~300 Mbps on the 5 GHz band. That being said, you won't get these speeds accessing the TC's HDD as the hardware Apple uses inside the Time Capsule creates a bottleneck.