There’s no right answer on the hardware requirements, but just about any modern PC that can run windows 95, 98, ME, XP, Vista, or 7 will work just fine. A lot of people are using atom powered systems (i.e., netbooks). You can find a very active forum for hobbyists at pbxinaflash.com.
If you want to make outgoing calls, you can subscribe to a SIP trunking service, or you can connect a standard telephone line to your system using an FXO device such as the Grandstream GXW-4104 or the OBI 110.
If you choose a SIP Trunk service, you’ll need a DID (a phone #) if you want to receive incoming calls. You don’t need a DID if you only want to make outgoing calls.