PowerBook G4 - OS 10.3.9 - weak internet connection

I live in the Netherlands, I have two laptops which are connected to the internet through a FritzBox 7360 modem/router (no AirPort station in between). My MacBook (OS 10.6.8 ) connects to the WLAN without any problem, but my PowerBook G4 (OS 10.3.9) from way back 2003, frequently slips out of the connection. I intend to buy a new iMac shortly to replace the PowerBook but I am curious to know the reason why its connection to the internet is so weak. The problem did exist before my recent house moving, but has increased in my new situation. Is it possible that it is caused by a wall of heavy concrete (steel inforced or how do you guys call that) and some other less heavy) walls between the router and the Mac? Is the antenna in the PB weaker than the MacBook antenna? Can I expect similar problems with the new iMac that I want to purchase? How can I be sure before the purchase is made? Thanks for your help in advance