I have a MacBook Air and a Dell Inspiron E1505 notebook; as well as a DIR-615, obviously.
Well... Whenever I have JUST my MacBook connected to the wireless network, I get my full bandwith, and AirPort stays connected to it. But, when the Dell connects to the network, I stay connected, but I can't access the Internet. If I try to load pages, it tries to request the page, and it won't ever load. Then I lose my connection to online apps such as iTunes Radio, and instant messengers.
Its a very hard problem to describe, but I hope the above description kinda covers it.
I have re-flashed the firmware, and I have restored defaults. Nothing fixes it.
Not to mention that I can't use WPA, because it enables Wireless-N mode, and then my Mac will have no Internet connection at all, but it will connect to the network.