In one forest, I have a Windows 2003 AD Integrated DNS as my primary DC. I then configured an additional DC on the same forest with it being AD Integrated DNS as well. My thinking is that on my client machines (XP/7), on the DNS settings in tcp/ip, the Primary DNS configured there is pointing to the Primary DC's ip address and the Secondary DNS is pointing to the Secondary DC's ip address. Since both DC's are also AD and DNS, does this mean that in the event the Primary DC fails, the client machines should still be able to login to the domain since I have a Secondary DC with AD Integrated DNS in which it will also authenticate all users on the domain, right? Highly appreciate everyone's advice.
Yes, that's what it means. Assuming your second DC is actually set up properly, if your clients can't reach the first DC, they will try the second one... and assuming the second one is reachable and properly configured, they'll be able to log in. In that vein, make sure the second DC is setup as a Global Catalog.
P.S.: Active Directory (post NT 4) is set up as a multiple-master configuration, so there's no such thing as primary or secondary Domain Controllers anymore. The proper term is Primary Domain Controller Emulator (PDC Emulator). See this Microsoft support article for more details.