Firewall PC

Hello,

I have 5 PC's in my net work at home. All are connected to the internet
via a Sitecom WL-114 router with DHCP. On three PC's I'm using Suse 9.2
pro. My intention is to make one PC a Linux firewall PC and that's why
I'm posting this message.
My questions are:
the firewall that I want to configure as a firewall PC is connected to
the internet via the WL-114 router and gets its IP-address from this
router. I bought a switch to place between the PC that will be shielded
by this firewall and the firewall PC (I put in a second 10/100 network
card). My first question is (since I found out that the switch has to be
returned by me because of powerfailure): is this also possible with my
old non-wireless Sitecom router or do I have to use a switch instead of
a router?
I connected both PC's to the old router using straight (UTP) cables. My
second question is: is this correct or do I have to use cross-link cables?
If it's possible to use the old router do I have to push the outside
button so it functions as an uplink or do I have to make it function as
a real router?
And my last questions concern this: do I have to make the firewall PC
function as a DHCP router or do I have to set up the second network
card with a fixed IP-address. What IP-address does the shielded PC get?
And what must its gateway be: The IP-address of the WL-114 router or the
IP-address of the firewall PC.

Tnx in advance...