It won't be possible to upgrade a Small Business Server 2003 installation to one of the new 2008 editions. You'll have to perform a migration to 64-bit hardware.
It's all because of the Exchange 2007 component included with SBS 2008. Since it's 64-bit only, the Windows 2008 version must be 64-bit as well. And because of that, you won't be able to insert the install DVD and have it upgrade the existing 32-bit OS. (Even if it worked, you really wouldn't want to do that on a server anyway.)
More on migrations and SBS 2008 to come.