How Many DNS Servers??

I run three DNS servers now. One is internal and two are outside the firewall. I haven't seen any adverse affects as yet. Just make sure that your internal DNS Server is on the top of the list like you stated above or your clients won't utilize the internal server

Move your DNS server two the 'secondary' position and one of the ISP (their secondary) to the third position (I guess you'll no longer have the third one in your Scope Properties...)

An internal DNS server is common. The main reasons to have an internal DNS server is to enhance access to Exchange servers, Unix machines and Intranets. You should put the internal on top of the list, for since your DNS, likely, will have fewer entries, if what the client request is not on this list it will just move on to the external ones. On the other hand, if the request is for something internal, the request will not go out to the internat and back.

Good luck!

I guess you have a leased line to your ISP, because otherwise you would not rely only an DNS Server from "outside". But: having an internal DNS on top of the list of your DHCP properties greatly reduces traffic that would go to your ISP and back to the client just for a lookup of an internal resource. So your internal DNS should be the first, and is obviously the one with the fastet response time. I would make your own DNS server to forward unknown requests to your ISP (check "use forwarder" set time to 4 secs), like this it is not up to the clients to go to the next DNS in the list if there's no response. You could even decide to make your server acting as a slave server of your ISPs server (replication in intervals).