DNS Namespace Setup

I'm currently in the process of loading a new DC with SBS 2000. The current internal DNS name is "store." with no namespace declared such as .com, .local, etc. I read the prior implementors notes and they state that it was done this was on purpose, but the only reason stated was to avoid the use of .com. The implementation was less than ideal, so I'm wondering if this was an uninformed conclusion to simply avoid the use of .com. I have since read that not declaring a namespace can cause some unique problems, but no examples were given. I would like to clarify this - whether a [DOMAIN NAME].[NAMESPACE] is required or if only a [DOMAIN NAME]. is sufficent or even ideal for specific situations. Also, can the namespace portion of the full DNS name be changed on a domain controller (or if a full reinstallation is required).
FYI, half of the clients are Win2000 and the other half are DOS, which uses NETBEUI. I doubt that will have DNS setup implications, but thought I'd mention that anyway. Thanks