Since his old WLAN-card broke, my brother bought a "new" D-Link DWL-520+. Plainly spoken, this card is crap (at least concerning transmit distance). So, another solution for connecting him to our network had to be found. Fortunately, a friend of mine sold his Linksys WRT54G for approximately half the price we had to pay for the D-Link DWL-520+...
I quickly flashed the router with the DD-WRT firmware, entered the proper IP addresses and chose client bridge mode. And lo and behold, it works flawlessly. As my brother just set-up an old computer that lacks a wireless card, thus connecting to LANs by wire, the WRT54G came just in time and we literally killed 2 birds with one stone.
Moral(s) of the story