I am replacing all of the wiring in my old home. I am using a 20a breaker with 12-2 wiring. I'm pretty sure that I have to use 20a outlets and switches as well right? I ask because the 15a outlets and switches are less expensive. I am also confused because the 20a and 15a parts both opperate at the same voltage, but different amps.

Well, first off, voltage is not amperage, and vice-versa. Amperage is the size of the circuit.

Unless you are in Canada, you CAN Use 15a receptacles on 20A circuits. The only exception is if you have only one receptacle. By this I mean one simplex receptacle. One standard duplex receptacle is two receptacles on one yoke.

Switches are depended o the load. If the load you are switching is 15A or less then you can use 15A switches.