Sorry, I missed this message.
You're totally fine with WSS3 or MOSS with Office 2003. You'll get much BETTER integration when you update the client (Outlook 2007 is particularly INCREDIBLE against WSS3/MOSS2007), but the old version will continue to treat the new WSS/MOSS installation as it did the old one.
Don't go through SPS2003... why bother... If you NEED the MOSS capabilities, go to MOSS. Look carefully at cost & governance plans. Of course MOSS provides functionality against several areas (collab, search, BI, forms, portal...) but you're not going to use them all at once, I'm guessing. Figure out whether it's worth the cost at this moment to go to MOSS, or whethe WSS (plus, perhaps Search Server 2008 Express, available VERY soon) will do the trick.
Dan Holme
Office & SharePoint Pro Community Leader
MVP (Windows Server - Directory Services)