Ray Fleming has very kindly created a blog entry with all you need to know about the different versions of Windows 7 and the different ways to purchase the,.

Check it out at Microsoft UK Schools News Blog : What features are in which version of Windows 7?

BTW - Some of the best reasons for using Windows 7 are BitLocker/BitLocker to Go, DirectAccess and virtualisation rights. You'll noticve that to get these and volume activation you need Enterprise which basically means you need Software Assurance.