Windows

You can leave it as just DOMAIN. You don't necessarily need the dot com.

If necessary, you can change it later, but not without jumping through some hoops - or should I say, windows.

If you need to rename it later:

How to Rename the DNS Name of a Windows 2000 Domain

http://support.microsoft.com/default.aspx?scid=kb;en-us;292541

To expand on maxwell's fine answer I would say leave
the name alone, but with a few notes:

If your domain has multiple controllers then upgrade
the PDC first. Also, if you do add an extension then do
not use ".com" unless it is a public domain that is
registered. if the namespace is for internal only then
use something like ".local".

Good Luck,

razz