I have never installed an IDS. Another one of things I keep saying I am going to try and never get around too. :-/ I do trust my firewall though. But it does add another layer of security.
I don't know about that. The advantage is the firewall box is the only box exposed to the internet and visible to a hackers eyes. Besides it gives you some networking experience. ;DAlso, when setting up a firewall i have heard that it is bad practice to set up your firewall on your main box as this can be beaten quite easily through port 25.