Is your WAP set to both b and g? If so, set it to g only.
You will find that 50-60% of theoretical max throughput is considered good, due to contention (it's a shared access medium), protocol and security overhead.
As a test, try removing security (WPA) and seeing the effect. If your D-Link (and / or your WNIC) is not very fast at encryption, you will notice a difference. I'm not suggesting that you leave encryption off permanently, BTW!
SD