A few small questions
 

I got a Linux box that I have currently been using for just samba shares for my home network, well I am getting board and was thinking about building another Linux box to act as a firewall, domain Controler, and DNS. This just for a home network, fun and learning.
I have a Linksys router and was wondering if I would get any benifits from having a fire wall for stuff to go though before hitting the internet ?

also on the phsyical side of things would the configuration be Cable Modem---Firewall---Router---Switch----PCs
or
Cable Modem---Router---Firewall---Switch----PCs
?

Can I host a Private Domain in my house with just a Domain Controler and can Linux serve as such ?

I don't think it makes a huge difference, it's just important that the firewall is between the net and your pcs. Of course it's good to try and block unwanted traffic as early as possible, to reduce unneeded network traffic. Just out of curiosity, why have a router and a switch connected to each other, and a separate cable modem? Sounds like some of the devices are doing the same jobs after each other..

Well why not? It depends what you want your "domain controller" actually do, but in any case Linux can probably do it, it's just up to what you want --> what you need to do to get there.

I have a 5 port router and tomany things that require internet access X box, X box 360, Linux Server Wifes PC, my Normal PC, and some times a guest pc. I didnt want to have a bunch of cables running over the place, so I used a switch to feed the front room (1 Computer, xbox and a guest).

almost forgot the cable modem only has a rg-6 connecter on it for the cable, and a rj-45 jack for internet.