I second (or third, I guess) the Coinbase recommendation. I've got an article that should help you get started with Coinbase, if you haven't already:
http://www.bit-buddy.com/buying-bitcoins-on-coinbase.html.
That doesn't specifically addressed the withdrawal of USD, but basically, once your Coinbase account is set up and your bank account is verified, you just sell your BTC, and your dollars are deposited directly to your linked account.
Coinbase charges a flat 1% fee every time you buy or sell Bitcoins.