awful wireless connection

[MacDorchy]

I have a 24" intel iMac, 2 years old, and a brand new macbook. I am using the built-in airpport to share my cable modem internet connection with the macbook. When I am using ONLY the macbook, the bandwidth speed seems fine for browsing and email and what not. But when I am playing World of Warcraft on my iMac and my wife is using the macbook, her bandwidth drops off to nothing. I used to play WoW on my old PC while my wife surfed on her PC laptop using a wirelss G router and she never had a problem with bandwidth...sometimes it would slow a little but not too badly. I checked the signal strength in the house using my iPhone and it seems fine when I'm logged into WoW, but her bandwidth is just terrible. Could it be that I need a range extender, a better/faster router, or, is there a way I can change settings on the Airport to better share the connection? I don't think that the cable modem connection is too slow, it's the standard Comcast high speed connection. I'm very frustrated! Would an ethernet wired connection help? Any ideas?