Hi,
I think that I've figured things out.
First of all, from talking to Buffalo tech support, it appears that the WHR-G300N only operates @ 2.4Ghz (and not at all @ 5Ghz).
Then, I found this page, regarding the Intel 4965AGN chipset:
http://www.intel.com/network/connectivity/products/wireless/wireless_n/overview.htm
Note that on that page, under "Friendly Neighbor Assurance", it says:
"Supports current Draft-N standard direction to support 40MHz channels only in 5GHz
spectrum to minimize negative impact to legacy devices/networks nearby."
In other words, it appears that the 4965AGN is purposely designed to prevent it from using the 50MHz channels @ 5Ghz, and thus it can only interoperate @ 2.4Ghz.
From other searching that I did, 11n is suppose to be able to operate @ 300mbps, but apparently it's more difficult to achieve that @ 2.4Ghz, so that's apparently why, in spite of enabling 40MHz operation on the WHR-G300N, I wasn't able to get a connection greater than 130mbps.
To kind of prove this to myself, I did more testing, with a Belkin F5D0813 11n PCCARD that I had, in a different laptop (my DV9930US only has an Expresscard slot).
The Belkin adapter uses a Ralink RT2860 chipset, so I installed the driver and management app from Ralink:
http://www.ralinktech.com/support.php
Then, I tested again, using the Belkin card.
The nice thing about the Ralink management app is that it shows receive and send connection speeds along with "throughput".
When I tested with the Belkin card and the WHR-G300N, the Ralink app showed that I was getting ~270mbps and ~150mbps receive/send (again, this was on 2.4Ghz)!!
So, mystery solved....
Anyway, I hope that this information might help someone else...
Jim