To utilize the gain in "N" signal strength, both the router and computer network adapter need to be "N" device.
So adding an "N" USB adapter would allow you to use the "N" signal.
We installed a Linksys WRT300N router in a business.
Our laptops were "B&G" and were constantly having connection issues.
I reconfigured the router to to turn off the "N" features and use "B&G" only.
Wireless signal was awesome after that change
Maybe it was the goofy antenna on the WRT300N.