NT4 to W2K migration

What are the differences between setting up a single master domain model with 3 OUs and a single master domain with 3 child domains? At present we’ve got a NT environment – 3 domains – 70 users. If I decide to go for the 1 domain 3 OUs option can the machines in the OUs keep their existing TCP/IP settings (some are in the 1.0.0.1 – 1.0.0.254 range and others are in the 1.0.1.0 – 1.0.1.254 range)? Please advise what my options are for upgrading to a Active Directory environment?