G'day Bytehd,
It sounds like you are about to have alot of fun (you may not agree with me at this point in time)
First of all make sure you backups have been successful for a while. Especially if this is you only box. I would also make sure you have a copy of ERD floppy disks as these are good to have around if you need to repair the restored server.
Another thing is if you have a 2003 DC, it won't allow you to install TS in application mode for users to access. Though this is a pain it is actually very good practice.
You will also need to prepare your AD for the upgrade if you plan to upgrade AD at the same time (i am assuming this is your only DC in your site as there has been no mention of any others). You will need to run adprep /forestprep and adprep /domainprep.
**If you intend on saying at the level of AD you are currently at, this may not be required.
If you are upgrading your AD, you will need to make sure that the DC you are upgrading has all of the FSMO roles.
When ever I upgrade something like this at a customer site, I always perform a health check of AD. I do this by running some of the tools that MS provide to do this. The 2 I generally use are DCDiag and NetDiag
E.g. DCdiag /v >DCDiag.txt
This will identify everything on your domain (FSMO roles, GC's, DNS health, replication health etc) and more importantly any problems.
The below link is quite heavy reading, but I recommend you use it to prepare.
"Assumption is the mother of all f#%kups!