FastLaneJB just showed you how. Enter what he said at your shell prompt, but minus the --dport 80 script and it'll allow your XBox to be open to every port. That's leaving yourself and your clan pretty vulnerable to malware and smut though, especially if your DHCP server assigns those IP addresses the next time around to any PCs on your LAN. Obvioulsy you would use the ip addresses of the game units in place of what he posted. Another potential problem is the line number placement of your rule...it could be placed after any redirect Squid might have on your nat table and negate the rule. Better to include line numbers and mac addresses and then if it works for you, place the script in /etc/rc.d/rc.firewall.local so that it's included anytime your firewall is rebooted. Otherwise it'll get wiped every time.
iptables -t nat -I PREROUTING 1 -m mac --mac-source E0-C9-4T-B8-B4-99 -j ACCEPT
iptables -t nat -I PREROUTING 2 -m mac --mac-source C5-T7-3E-D5-FF-09 -j ACCEPT
Two boxes hence two mac addresses. Placed at lines one and two every time, prior to any Squid redirect on port 82.
If you need to revert back to the original iptables, prior to entering anything in /etc/rc.d/rc.firewall.local, just restart the firewall service and any changes will get wiped.
service firewall restart