Hi,
sorry , I kinda overlooked this, so that is quiet simple, you only need to do the following actualy :
CODE
echo "1" > /proc/sys/net/ipv4/ip_forward
then you can put this somewhere where it will automaticaly launch , probably /etc/rc.d/rc.local
now try to ping external interface of the Suse machine , that should work, try to ping anything else no go, do this to make that happen .
You'll need to add this to POSTROUTING in order to transulate packets from internal lan to outside , and back.
CODE
iptables -t nat -A POSTROUTING -s 192.168.0.0/255.255.255.0 -o $EXTIF -j SNAT --to-source $EXTIP
$EXTIF = external interface
$EXTIP = external ip address
Hopefully you already have some sorta firewall rules in action. You can type both of these commands into the console as root, and the effect should be imediately available.
Then you config WinXP computer to use internap ip of your Suse computer as Gateway, and configure it to use ISP's DNS server. (no real point in making your own caching only dns server for 2 computers).
Hopefully this will help

Sincerely
Robert B