When you describe your situation, you're not always being clear and using pronouns which are somewhat ambiguous. We also have no idea if you are talking about in place or moving to new machines. Be careful of that.

You do need SP4 on SQL Server 2000 for an upgrade.

According to this, http://msdn.microsoft.com/en-us/library/ms143506%28v=sql.100%29.aspx, you need SP2 for Windows 2003.

However, if you have a new SQL Server 2008 instance, and are moving the files to that instance, why upgrade? You can restore a SQL Server 2000 (SP4) backup on a SQL Server 2008 instance.