I have the same problem but I don't think it's the D-link router, rather I think it's something with wifi in general. I have tried a total of 3 different wireless routers and the best throughput I get is 2-3Mb/s using wifi. I've tried turning off QoS & WISH but it makes no difference.
If I use ethernet cable i get 40-45Mb/s throughput with all 3 routers.. (to a synology NAS).
Perhaps it's something in the OS but again I've tried Win XP and Win Vista and get the same speeds..
If you find a solution please post!
//Graham