Best way to keep address books in sync?

I have my contacts on 2 machines I use (one Mac and one PC). I use Outlook 2003 on the PC to store my contacts and OS X Address Book on the Mac. I want to keep them both in sync. My problem is when I export my contacts on the Mac, it creates one file (named vCards) which when I try to open it in Outlook 2003, it just shows the first contact in my list. Anyone else in the same situation? What do you do?