stick is correct, install a bitrate measuring plugin, and you will see that its usually > 6Mbps (unless you are looking at a package/channel with v low quality then it could be as low as 2Mbps) , 6Mbps is the limit which wireless lan protocol b can handle, it is 11 Mbps but 5 of this 11 is taken up with other things such as CRC,anti collision,etc, the maximum data throughput is 6Mbps, if you update to wireless G you will have no problems at all (data throughput > 20 Mbps).
so sorry dynamo, no plugin will help you, its a hardware limitation