If you already have a domain, then you must already have a domain controller. Assuming your currently running NT on your domain then DNS is not a must have as NT uses WINS.
If and when you go for AD then you must have DNS installed and running. However, AD DNS is different to NT DNS (which doesnt support some records required by AD) so i'd suggest upgrading your current DC to 2000/2003 which, during the installation, you will be prompted to provide (or install in your case) DNS. The installation will install and configure DNS for you.
wn i gro up i wna b a typo