Microsoft has put up an online "Beta 2 Test Drive" for people who are on the fence about upgrading. In case you don't want to install the actual Beta for fears that it'll interfere with your older version of office, this preview lets you check out what's new—and what's not. All you need is IE 5.5 or later and the Citrix Browser Plugin.
Try it out then come back and tell us whether there's enough new stuff there for you to pony up a couple hundred bucks to upgrade from Office 2003.
Test Drive [Microsoft]
Microsoft launches online Office 2007 preview [Ars Technica]