Hi,
I would guess that most people, including myself at both work and home, are using Excel 2003 (or earlier) but obviously over time more and more will upgrade to 2007.
If you don't have either there's no real benefit to buying 2003 as you'll be missing out on the new features introduced in 2007, i.e. extra rows/columns, extra conditional formats etc
You might find that some VBA that you've got in old spreadsheets caused problems, particularly when manipulating toobars, and a lot of things I've read slag of the new interface but I guess you get used to it.
Dom