Domain names !!

Hi,

My organisation has 2 NT4.0 networks at the moment. One is admin and the other is finance. We are planning to upgrade to the windows2003 server network. What would the domain name be after we upgrade the 2 networks. Would they be admin.com and finance.com after the upgrade or do we have to create a root domain called mycompany.com?

How easy is it to upgrade the whole network to windows2003?

Please suggest