In Xandros when you run /sbin/ifconfig
do you have a connection called ppp0? The following assumes that you do.
The following will configure your other network cards and add NAT router for ppp0. Note that you will have to force netmask 255.255.255.0 on every device as the 10.0.0.0 range is considered a Class A network and defaults to netmask 255.0.0.0
ifconfig eth1 10.10.0.1 netmask 255.255.255.0
ifconfig eth2 192.168.5.1
/sbin/iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE
echo 1 > /proc/sys/net/ipv4/ip_forward
On the wireless router you'll need to disable the PPPoE and DHCP settings, set the router "public" IP address to 192.168.5.2 and configure a default route of 192.168.5.1. Warning: this may not be possible on some routers. You can leave it doing NAT - set the DHCP address pool to 10.10.1.2 - 10.10.1.254 netmask 255.255.255.0.
Set LAN clients up as 10.10.0.2-10.10.0.254 netmask 255.255.255.0, default route 10.10.0.1, ISP nameservers for DNS.
Wireless clients should pick up an IP address from the router as normal. Use ISP nameservers for DNS.
Once you've verified everything works, you can incorporate these settings in your startup files and maybe add a DHCP service and caching name service on your box.