TechReviewSource.com posted a comparison of the Windows 7 Product Editions:

Windows 7 Home Premium:
Home Premium will be the edition that an overwhelming majority of computer users in the United States will choose, mostly because it will come pre-installed on new computers after the October 22nd release of Windows 7. This edition comes with all of the desktop enhancements that Windows 7 has to offer, including Windows Aero, Flip 3D, Live Thumbnails, and the ability to customize all aspects of the user interface. Windows 7 Home Premium comes with all of the multimedia features, such as Windows Media Center, DVD Maker, Windows Media Player, and all of the games that Windows 7 has to offer. Windows Mobility Center is also included, which provides advanced features for laptop and portable devices as well as touch input and multi-touch functionality. If you are an average to above-average computer user who will use this product in a home or home business setting, this is most likely the edition that you will want. It is priced right at $120 for an upgrade version for current users of Windows XP or Vista and it will definitely run on any computer that decently runs XP and Vista right now.

Windows 7 Professional:
Users of Windows 7 Professional will be small to mid-size businesses and companies who need advanced networking features, such as Active Directory and domain log-in support. Professional also offers network backup, an Encrypting File System, and the ability to connect to and control a remote PC via Remote Desktop. XP Mode is also available to Professional edition users, which allows users to install and run applications that only work in Windows XP in a seamless Windows 7 environment. Professional edition also supports a larger amount of RAM: 192GB (64-bit) and 16GB (32-bit)
The biggest two questions of whether or not you need Windows 7 Professional are the following: Do you need domain support and do you need seamless backward compatibility for legacy Windows XP applications?

Windows 7 Ultimate:
This edition is for the power-user and computer enthusiast. It includes every single available Windows 7 feature, but is still relatively similar to the Professional edition. Windows 7 Ultimate gives users access to BitLocker and BitLocker To Go, which is a drive encryption system for the system hard drive and portable media, like a USB thumbdrive. Home users will not need any of these more advanced features and the price of $100 extra is most likely not worth spending for it.