Make sure you have the latest drivers from:
http://www.wireless-d...less-drivers-utility/
My brother has had a similar problem recently. His Virgin Fibre-Optic cable terminates just outside his home and the signal comes through to the house on a copper cable. His plugged his Windows 7 laptop directly into the wireless router and got 60MB straight away. But when he tried to connect wirelessly to his XP PC upstairs, the signal dropped dramatically. He needed a PCIe card like yours to get up to speed and it did.
I suspect that the problem starts at the router and the way you're connected. The router should be the same 802.11n standard and should be connected directly to the Master socket and, which is the first point of entry from the cable outside and not elsewhere in the house via an extension lead socket.