Well, you can go to a cloud provider like IBM Softlayer or Amazon, but that would mean putting everything in the public cloud.
The other choice is Linux. That would depend entirely on what you are doing on the WS2003 boxes. If it's file and print or simple database, you can do it fairly easily. If you are using a highly vertical Windows app not found on Linux, things become more challenging. You ability to move will depend on the degree of Windows apps you use and if there are Linux equivalents. If you have a lot of home grown apps, you will have a real challenge ahead of you.