Well, to be fair, you can immediately forget about the Starter version (it allows only three concurrent apps), the Home Basic version will be available in developing markets only, and Enterprise is for enterprises.
So that leaves only three versions you have to choose between.
There are only two mainstream versions:
Windows 7 Home Premium - (Widespread retail availability)
Windows 7 Professional -(Super set of Premium, widespread retail availability)
Unless you need "Domain join, Remote Desktop host, advanced backup, EFS and Offline Folders", you don't need Professional. Additionally, you no longer have to buy Ultimate if you want Remote Desktop and Windows Media Center. You just need to buy Professional as it's just a super set of Premium.
Mainstream consumers will never see these versions:
Windows 7 Starter - (Developing nations only, no retail availability)
Windows 7 Home Basic - (Developing nations only, no retail availability)
Windows 7 Enterprise - (Volume licensing only, no retail availability)
Windows 7 Ultimate - (Super set of Professional, Limited retail availability, marketing limited)
Microsoft's Windows 7 marketing directive specifically tells partners not to market Ultimate to home users. Unless you need "Branche Cache, MUI language packs and boot from VHD", you don't need Ultimate.
The starter is available to third world countries, Home Basic is for anyone, I have it running on a Core Duo, 1gb RAM machine (runs like a dog). It's basically Vista without the see through. The XP home equivalent of Vista.