What information do you want to sync? Calendar appointments? Tasks? Notes?
Regardless of what version of Office 2008 you purchase, you may encounter serious problems with syncing functionality. I own an iPod Touch and have had zero success in getting sync set up between the iPod Touch and Entourage. I think you will find dozens and dozens of similar complaints. Entourage does not offer a dedicated sync conduit with Apple mobile devices - it uses Apple's iSync functionality as I understand it, which has many reported problems and bugs.
You may be able to use MarkSpace's product "The Missing Sync" to sync your iPhone's notes with Entourage. You might also have some hit-and-miss success with syncing your calendar with iPhone to iCal via iTunes, and then syncing Entourage to iCal. I have never gotten it to work satisfactorily, however, and many people have reported problems with this. I have also tried Spanning Sync (a third-party product) to sync my iCal data with Google Calendar and then the Entourage calendar with iCal, but again, mixed results.
The fact of the matter is, you will not be able to sync certain information between iPhone/iPod Touch and Entourage, no matter how hard you try. They are two different products. You will be unable to sync categories or Entourage project data, for instance.
Basically put, I would advise you to forget about syncing Entourage data with your iPhone, unless you have a lot of time to waste, or wish to spend more money on third-party products that may or may not be satisfying. I use Cultured Code's Things to manage my tasks on the iPod Touch, which syncs very well with the OS X standalone app. I use iCal to manage my calendar data, which does a fine job of syncing with iPod Touch calendar data via iTunes. Basically put, I've chosen to leave Entourage out of the picture. Its import/export functionality, lack of a dedicated sync conduit, and lack of cloud storage capabilities makes it a poor partner with Apple's mobile devices.
Your mileage may differ, however. If you have the time and money, experiment and see if something works for you. Just don't expect it to work out of the box - it isn't designed to.
Jeff