noob questions regarding domains/subdomains/initial setup

Hello all!

First off, yes, I'm very new to all of this. I do basic tech support at an ISP that has graciously allowed me to set up my own server. Of course, I installed ISPconfig immediately as it seems very user-friendly.

I had no trouble getting it installed with CentOS, and even had a few test sites running on it using domains from no-ip.org to test.

Now I'm ready to place some real content on this baby, and realized that I have no idea how to configure it the way I want!

Heres what I'd like to do:

I currently have a domain that is hosting a website on a different server (www.thatjoshguy.com) I would like to migrate that site to my server. (I know how to do that)

I have another domain with another site on the same server as before (www.shrevefamily.com) that will also be migrated over.

That part is simple enough... what I don't understand is this:

I'm going to be installing Drupal and Gallery2 as multisite instalations.

I want to be able to access the server at "server.thatjoshguy.com"
I want to be able to access the Drupal Core at "drupal.thatjoshguy.com"
and I want to be able to access the Gallery2 Core at "gallery.thatjoshguy.com"

Now, currently, the domain thatjoshguy.com's nameservers are my ISP's. i know that ISPconfig can serve up some DNS entries for subdomains, but I have no idea how! What do I point where? I've looked through the forums and whatever tutorials I can find on the web, but they may have just confused me even more.

Would it be easier to register a new domain specifically for the server such as www.mydomain.com, and then host www.thatjoshguy.com and ww.shrevefamily.com as regular sites, and set up subdomains for the drupal and gallery installations such as drupal.mydomain.com, gallery.mydomain.com? What should my ns1 and ns2 entries be for the domain and my server settings?

Would anyone be so kind as to lend me a hand? Thanks in advance!