WAN design

I am writing a paper on designing a WAN. I understand the concepts of a WAN, but I have never designed one before. I have setup VPN links to create a WAN-like network, but those were just with cable or DSL and between two sites. If I have three or more sites, how would they connect? What hardware is required? Are there any websites about building WANs? (I Google?ed, and found some very basic WAN sites.)
Without knowing any site location particulars can I just say I would use Frame Relay? (Each of the sites would be in a major metropolitan area.)
Can anyone point me in a direction for more info?