Trying to set up a router, I like to do one thing at a time though, so here's my question: Forgetting about security, and NAT for right now, All I want to do is have my PC's on the internal side of the router be able to ping the modem.
The setup is:
DSL-Modem(192.168.1.254)---->(192.168.1.1)eth0/Gentoo\eth1(192.168.0.1)---->switch--->PC's(192.168.0.x)
Im sure my kernel looks good, I know that /proc/sys/net/ipv4/ip_forward is set to "1", and is so everytime I boot, I know the iptables config file in /etc/conf.d/iptables has ip forwarding enabled.
With all that said, these basic rules should allow me to ping the router correct?
iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT
iptables -A FORWARD -i eth0 -o eth1 -j ACCEPT
iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT

I dont see why I would need anything else to just ping the router. Please tell me if Im missing something or you see something wrong. RIght now its timing out every time.

echo 1 > /proc/sys/net/ipv4/ip_forward

To enable forwarding

I know, is there anything else that would cause this not to work?

Sorry for skipping that part, must have read your post too fast.

Try

iptables --flush
iptables --table nat --flush
iptables --delete-chain
iptables --table nat --delete-chain
iptables --table nat --append POSTROUTING --out-interface eth1 -j MASQUERADE
iptables --append FORWARD --in-interface eth0 -j ACCEPT

if your LAN iface is eth0. This at least works for me.

Well that worked for some reason, I didnt think I would need NAT to just ping, but I guess I did. I really appreciate the help.
Now that that is working, am I gonna need this line:
iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT
Or better question, would I have any problems with any services on the PC's right now?

Nat is needed when the two ifaces are not on the same subnet.

Just nice to help.

Not on your gentoo, but here is how to open ports to other comps behind it:

(one line)
iptables -A FORWARD -i eth1 -p tcp --dport 80 -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT

iptables -t nat -A PREROUTING -j DNAT -p tcp --dport 80 --to-destination 192.168.0.x

This redirects to a webserver with the tcp protocol