Shaitan, as far as I know, you won't need to download a US-client to run US-servers. Just go into folder WoW\Data\EnGB(i guess) and open eralmlist.wtf with notepad.

In there, change the line:
set realmlist eu.logon.worldofwarcraft.com

to:
set realmlist us.logon.worldofwarcraft.com

And you should be set to join the US-servers.