When installing previous versions of SQL Server, I'd always keep a list of the exact privileges that a SQL Server service account would need. I'd make up a new user and give them exactly these rights. The SQL Server installation program would grant the appropriate permissions during the install, and I'd be set. Principle of least privilege, run SQL Server and associated as the minimum privilege account needed. If I thought about changing the service account, I'd call up knowledge base article Q283811 and sigh at the list of work items. And make sure that I used Enterprise Manager, as the article recommends.
Things are a lot simpler in SQL Server 2005. When you install SQL Server, the process creates Windows groups with exactly the minimum privlege that you need for the appropriate services. The user you specify during the install (which should be a "normal", low privilege user and NOT administrator or LocalSystem, is plunked into the appropriate group. There is a Windows Group for each SQL Server-related service, all nicely decked out with least privileges (file system DACLs, security policies, registry key access, etc). NO more privilege lists to keep around, although they are doc'd in BOL should you need to refer to them.
If you should need to change the service account, things are even better. Just put your new Service Account user in the appropriate Windows groups, and voila, no long list of tasks. Very cool. Or use Configuration Manager.
Twas' not all sweetness and light, however. an FAQ during the early days was "why does SQL Server setup "clutter" my directory with groups?" That's why. First time you need to change service accounts, you'll appreciate the "clutter".