I assume your firewall is running on your computer that is on the external interface. Are you allowing all incoming and outgoing traffic from your loopback device ??
$IPTABLES -A INPUT -j ACCEPT -i lo
$IPTABLES -A OUTPUT -j ACCEPT -o lo
as for allowing incoming ip's specifically for your 6000:60010 port range, by jsut setting your default policy on INPUT to drop will deny all external attempts to access it, if you want too allow people to start or send x related programs to other hosts, specify them
$IPTABLES -A INPUT -p tcp -s x.x.x.x --dport 6000:6010 -j ACCEPT
same as your example..
this will allow x.x.x.x to start a program on your computer and let them display it on theres, of course you have to also do
xhost +x.x.x.x and have your xserver set to recieve connections, i know debian turns this off by default, redhat leaves it on.