The names are just the beginning. Some of this is controlled by the world_default.xml file for each nation under the "Ethnicities" tab. By default, the United States generates players with 77% from the nation 0 nameset, 13% from the nation 1 nameset, 9% from the nation 39 nameset, .06% for nations 2 and 3, and .03% from all other namesets except 5, 6, and 33.
With your edited name files, you will need to edit the world_default.xml file (I recommend using Notepad++ to make changes, and backing up an original copy) to the ethnicities for your US nation.
This post gives a tutorial for what exactly you need to do, but the whole thread has much information for editing that file to anyone's liking:
http://www.ootpdevelopments.com/boar...3&postcount=33
(You can ignore the part about editing ethnicities.txt)