The differences are big, remember that if you are running vista then 2008 is ok, if you are running windows 7 you need to upgrade to 2008 R2 to get all the features.
Differences from 2003 to 2008 http://www.microsoft.com/downloads/details.aspx?FamilyID=173E6E9B-4D3E-4FD4-A2CF-73684FA46B60&displaylang=en
Differences between 2008 2008 r2
http://www.microsoft.com/windowsserver2008/en/us/r2-compare-features.aspx
