[ Connecting to XBOX Live via linux computer connected via wireless LAN ]

Things you'll need:

WLAN and LAN card installed
Crossover cable
Your WLAN interface is wlan0, and LAN is eth0
iptables installed

Open your terminal type ifconfig eth0 up
ifconfig eth0 192.168.x.x

cat /proc/sysnet/ipv4/ip_forward. If you get an output of 1 you're good, 0 run this command
echo "1" > /proc/sys/net/ipv4/ip_forward

Next Configure your iptables
iptables -t nat -A POSTROUTING -o wlan0 -s -j MASQUERADE. Note you may need to replace 2.0 use common sense.

Next go to the xbox dashboard into network settings. THen select IP address and set the config type to manual.

Making it permanent:

open the file /etc/rc.d/rc.local and put the following in:
ifconfig eth0 up
ifconfig eth0 192.168.x.x
echo "1" > /proc/sys/net/ipv4/ip_forward
iptables -t nat -A POSTROUTING -o wlan0 -s -j MASQUERADE

Finally enter these commands:

IPTABLES -t nat -A PREROUTING -i $WAN_IF -p tcp --dport 3074 -j DNAT --to-destination $XBOX_IP
IPTABLES -t nat -A PREROUTING -i $WAN_IF -p udp -m multiport --dports 88,3074 -j DNAT --to-destination $XBOX_IP
IPTABLES -A FORWARD -i $WAN_IF -d $XBOX_IP -p tcp --dport 3074 -j ACCEPT
IPTABLES -A FORWARD -i $WAN_IF -d $XBOX_IP -p udp -m multiport --dports 88,3074 -j ACCEPT