Hi,
One way I do this is to always have the latest in crucial software. Office is definitely crucial. Others depend on my clients needs and prefs, but Office is on that list for all.
In the corporate world, many are using 2003 but slowly going to 2007. So you should at least use software compatible with potential clients, which would be 2003. However, it's a better buy to get the latest.
I think MS has a deal right now that if you buy 2007 through them now, you'll get 2010 for free. So that would be my suggestion. Check
http://office2010.microsoft.com/ for latest info on that possibility.
One other thing, when buying Office, is to decide which version you'll need. Look for a comparison chart to determine difference in versions.
HTH!
Cortney