You need something like this
http://www.linksys.com/products/prod...d=38&prid=558.
Use it as a wireless ethernet bridge to Airport Express or any other wireless router. If you buy other wireless router with ethernet ports, you don't need additional wireless card for your Mac (plug your Mac into one of the port using wire). They are a lot cheaper than Airport Express.
Check out their user guide for setup help. Only problem to find out is whether it will provide good reception in the different floor where your game console is located.
The basic setup is
Airport Express to your DSL connection.
Wireless Game Adapter (Wireless ethernet bridge mode) to your game console.
Game console --> Wireless Game Adapter --> Airport Express --> Internet
Computer --> Airport Extreme Card --> Airport Express --> Internet, or
Computer --> Wireless router with ports --> Internet