Best wireless network card for pc

Hello,
I'm using a Dlink wireless network card for my desktop and accessing the internet through a 20MB broadband service. I believe I'm not getting the full 20MB (upload and download) when I surf. This is confirmed by the running the Test tool provided by the network service provider. Most of the time the tool is showing that I'm getting around 8-10MB for both download and upload. I've run the tool on my Dell laptop also running windows 7 and the Dell laptop is showing it's getting the full 20MB for both download and upload. Both desktop and laptop are next to each other and at the same distant from the router.

Can someone help me explain why my desktop wireless is not performing as well as my laptop's? Is there a better wireless adapter for my desktop?

Appreciate your help. Thanks