Not too sure what you get with 2003 but i think the additions in 2007
include

Write your blog in word and post straight to it

Take sharepoint lists and libraries offline in outlook 2007

Subscribe to site / blog rss feeds and receive them straight into
outlook (i use this all the time and its fantastic)

Excel services only really works with excel 2007 workbooks (natch)

If you have word 2007, look at file | prepare - its got all the doc
management stiff, especially marking a document as final

Powerpoint 2007 will natively use the sharepoint slide libraries

The point is, office  2007 radically extends moss - your question isn't
whether you should roll it out but whether you should roll it out before
upgrading the portal

The answer, I would think, depends on business need and whats driving
the decision - if I had the choice its hard to say as MOSS is such a
massive improvement over sps that I don't think you should let anything
hold you back from going to it but equally office 2007 adds so much

I guess, if you held me to it, i would go moss first then office as
users will see massive improvements straight away and get additional
benefits when they go through the office upgrade  so each time you do
something they can see the benefits they receive straight away