OK, you should right about now sacrafice a small fury woodland animal of your choice to the folks that developed ADMT at Microsoft.
Personally unless you have a solid reason for inplace upgrading, I would start out with a new Domain and Win2003 install. While there is a fairly even split of those of us who like either in place or parallel turn ups, I would challenge you to ask yourself why either option is the one you should choose.
I'd also recommend that unless you've done the 2003 training that you engage a VAR for professional services. Also beware the Exchange upgrade and make sure you're ready for it.
Just remember 2003 is locked down from the get go, you'll spend a week or two just loosening the securities depening on your users access and experience.
Also lean about GPO's ... you can migrate 90-100% or your logon scripts into GPO's.
Think about and document your idea's on the OU Structure as well. Have that in place prior to starting. Matter of fact, creating and managing OU's is a perfect Jr. Admin or Help Desk task!! So think about that as well.
Evaluate you're current security groups, then review the new options available to you in AD (domain and global groups) and associate that to the OU Structure.
I'm finding most customers are designing OU/AD Structure based on a few things. Usually it is Geographic dispersal or by Business unit organization. I've got one customer who's doing both. This just demonstrates the flexiblity of AD.
Remember with AD's ability to present more than 3 million objects you can easily overwhelm yourself and our peers if you don't apply the KISS rule!!!!
Here is one basic OU structure layout
joesnuff.com
_____________
joesnuffy.corp
Users
Computers
Servers
All the other default OU's
Atlanta
Support
Computers
Users
Groups
Sales
Inside
Computers
Users
Groups
Outside
Computers
Users
Groups
Portland
Executive
Computers
Users
Groups
HR
Computers
Users
Groups
You will notice a reoccuring theme with the base sub OU structure of Computers, Users and Groups. The first two are obvious, and the groups OU can be used for Security groups, Distribution list groups etc. You can also use this basic type of structure to delegate control easier. Say you want you're e-mail admin to manage all of the security groups (because with AD intergration, Exchange 2k and 03 both reference the security groups if you've given the group an addy) so the Mail Admin would only have rights to the Groups OU's in each of the OU's. Also this lets you be more specific with your GPO's. You can use the Computer portion just on the Computers OU and the User portion on just the Users sub OU's ..
Hope that helps and look around this site. There are some really good documents you can use for post and pre migration documentation and planning.
Don't skimp on this effort, once you impliment an inplace migration you'd play hell going back!!! Yet another reason for a parallel upgrade.
