I used Office 2003 Pro for quiet some time and I love it. However, I did receive a free copy of Office 2007 Pro through my school and I finally installed it. What I have to say:
Office 2007 Applications take longer to load and do simple functions on my computer (about an extra 8 seconds to load, 1-2 seconds for other tasks)
Office 2007 does look nice
Office 2007 requires more space for a similar install of Office 2003
In the end, I'll just keep 2007 since I already installed it and defragged.
I'd go with 2003 if I were you, It's like XP vs. Vista as noted before (2003 will be hella faster and have the essentials instead of fancy stuff =p).
Good luck