Problems forwarding ports

[Yohhan]

I have two computers set up on a home network, behind a linksys router. Whenever I set up the following ports to forward to 192.168.1.102 (to play online halo on the xbox), my other computer loses it's ability to access the internet. It still functions fine within the local network (I can ping the router and the working system that has all the ports forwarded to it), but i can't load anything from the 'net. Here are the ports that i'm forwarding (from gamespy.com)

6667 (IRC)
3783 (Voice Chat Port)
27900 (Master Server UDP Heartbeat)
28900 (Master Server List Request)
29900 (GP Connection Manager)
29901 (GP Search Manager)
13139 (Custom UDP Pings)
6515 (Dplay UDP)
6500 (Query Port)

Any help would be greatly appreciated.