If you are changing what name servers that have control of a domain you should use:
It's a fairly common activity to have registered a name elsewhere, but want to host the domain at dreamhost. If that's what you are doing, you wouldn't want to use IP addresses at all, but the names above.
Where are you being prompted for IP addresses of 2 names servers? It almost sounds like your making changes to force a local windows machine to use dreamhost DNS? This is something differnt than above, but sounds like you may be poking around in windows because Microsoft called them Primary and Secondary Nameservers and asked for IP addresses.
It's always best to avoid using numeric IP addresses because they can change. Half the point of DNS is so that network addresses can change at the will of network admin without effecting users. The other half of course is that names are easier to remember and use for all, admins and users alike.
Please tell us more about the specifics of what are doing and we can give better advice.