If you're unsure as to what the 5 different Macs are, here goes:
iMac: a desktop integrated into the monitor/monitor integrated into the desktop, reasonable performance
Mac Mini: ultra-compact desktop, very unexpandable, somewhat pokey performance wise.
Mac Pro: High-end Desktop/Workstation(uses expensive Xeon-CPUs)
Macbook: very light-weight notebook, 13.3" screen, average notebook performance
Macbook Pro: light-weight, high-end notebook, 15" or 17" display
Somewhat depends on how you define "unexpandable". All Macs, with the exception of the Mac Pro, have limited ability to "crack" them open to expand. All Macs, with the exception of the Mac mini, can be opened up to expand memory...at least without voiding the warranty (you can crack the mini to upgrade the memory as well, but you will void the warranty). The MacBook and Mac Pro can be opened to upgrade the hard drive...the MacBook Pro, iMac, and mini can as well, but you will void the warranty.
All Macs, however, can be expanded pretty well with USB and Firewire devices (all Macs come with Firewire ports...some even come with Firewire 800 ports). Without a doubt, however, the Mac Pro is the most "PeeCee" like in that it is a tower where you can easily add cards, drives, memory, etc.
[soapbox] Backup good...no backup bad!! [/soap box]