Know which Office 2007 apps you need and which editions include them? How about support for integrated Enterprise Content Management, electronic forms, or Information Rights Management (IRM) protection in a Windows Rights Management Services (RMS) network environment?
Microsoft has unleashed eight varieties of the latest Office suite, and mapping the options to your company's requirements might be a little tricky. To enhance the odds that you select an edition that includes all the stuff your users need without paying for a bunch of stuff they will NEVER need, here are a couple of extremely handy resources:
Which edition of Office 2007 is right for you? -- An in-depth look at which applications each suite includes, the retail and upgrade pricing, the capabilities and enhancements of the various applications (including the lesser-known ones, like Groove and Office Communicator), and a description of the aforementioned ECM, electronic forms, and IRM/RMS
A quick-glance guide to the Office 2007 editions -- A PDF chart you can use as a cheat sheet, which lists the most pertinent info about each edition
When the time comes to make a choice, these guides may save you some legwork. As member Apotheon says in a related discussion:
"When you have to make sure you get the right decision the first time because otherwise you'll end up blowing about six hundred dollars you can ill afford to waste on the wrong choice, suddenly the stress level of making decisions based on poorly documented choices like this can get pretty high."