How are your wireless settings configured? Please reference the following link to achieve 300Mbps.
Installed the vista 64 driver and am now getting 300 Mb
The Windows 7 driver is wrong it seems
I don't have a board that support PCIe. I have tested the DWA-552 and that is pretty much the same thing and works.
I just received a copy of build 7100 of win 7 and will give it a try.