Ausatco, without a detailed read of ORAC's link, I think it more or less gives the answer.
You don't need the AP900+ from D-Link. If you just put a DWL-520+ in the Desktop, you'll have an ad-hoc, peer to peer network equivalent to having a Cross-over Cat5/Cat6 cable.
But I'm finding that the penetration through the ancient walls of Fobotcso Folly is poor so I've invested in the AP900+ as a repeater. Haven't had time to sort out the configuration yet so have to resort to peculiar positioning of the laptop on the arm of the settee balanced on the Readers' Digest Atlas of the World. Polarisation of the Tx/Rx seems important. That sexy little aerial on the back of the card or Router Box seems to like to be on its side when talking to the laptop.