I currently have a domain w/in a domain. The main domain is already running ACTIVE Directory and is pretty much the master of everything. The smaller domain is running NT4 and is used for "in-house" TESTING and stuff.

My problem is in the NT domain I have a W2K server that I want to promote to domain controller, I pretty much want to get rid of the old NT4 box. What I want to know is once the W2k Server is promoted to domain controller and I turn on Active Directory will it AUTOMATICALLY get assimalated into the main domain that is already running Active Directory?

I want to get rid of NT4 but I still want the segregation that I currently have between the two domains. I hope this MADE sense.cant your test your theroy? but taking away the the nt4?Are they completely seperate domains now?
As long as you keep the NT4 domain (once you upgrade it to the 2K domain) name different from the 2K domain they will remain seperate.

You might look into making them both members of the same forest in the future, if you want them to communicate, yet remain seperate. However, if you want to keep them totally seperated, they can co-exist in the same LAN with different domains. 2000 is actually fairly picky about who gets promoted to Domain Controller (by requiring Domain Admin rights for your user account) and wont assimilate without you specifying that you want them to be.Yes, both are on two different domains but they can still see each other through network neighborhood.

I just want to be sure when I promote my current W2K server to a DC that it still will be its own domain. I never ran AD so I have no idea what I'm in for. I want to make sure my users can still login to the old domain name.Yes, you will be fine to promote it to DC.  It will take the PDC emulator role and everything will keep on chuggin.  They can see eachother in net hood because they are on the same network, there is nothing there that says they are actually in the same forest, thats more of a WINS and DNS thing than anything to do with AD.