Well you might want to install a second site to load balance the traffic. You can also dedicate servers to specific tasks. When you start adding additional servers you might keep in mind that you basically have a choice between few more powerful servers or a bunch of weaker ones. Half a dozen one the other.
Powerful servers cost more initially but can service more users.
Now might also be a good time to consider fail safe measures such as an uninterrupted power supply, tape backup or something on the high end like clustering. Linking sites together is accomplished with the use of connectors. Connectors do what the name implies. They connect one or more sites together to allow for seamless communication. This is almost like Exchanges native connector and is used to connect two Microsoft Exchange Sites.