Your first decision is whether you want to do in-place upgrades or rebuild them. Then you need to ADPREP both the forest and all domains in the forest.
In-place upgrading is realtively straight-forward, start off with the DC's doing the least. The trickiest one will probably be your TS licensing server, I haven't looked into in-place upgrades of these as at my company I'm doing rebuilds.
Rebuilds are a bit more complicated, ideally you need one new server to start with (unless you have a DC that's only doing authentication so there's no loss of service during the rebuild). As for in-place upgrades, start with the easier servers, MS recommend moving the PDCe and Domain Naming master roles across as soon as you have your first Win2003 DC - be careful with the PDCe though if you've configured the current PDCe for an external time-source, make sure you set this up again on the new PDCe.
After that just check out the MS KB articles on transferring stuff like WINS, DNS and DHCP.