Do you have to buy bitcoins first? or will btc-e allow you to buy litecoins with USD?
You can buy with USD. No need to convert to BTC first.
Assuming you can get your USD into BTC-e or Vircurex, that is. Neither is as easy as it sounds. BTC-e is notoriously hard to deposit to since half of their USD deposit options are shady russian sites and the others are exchanges that charge an arm and leg or force you to wait 3-4 days on a wire transfer (which costs ~$15). Vircurex only accepts VouchX as a USD deposit option, which is equally difficult to fund.
Your best bet is to buy bitcoins through an exchange like Mt.Gox/BitFloor/BitInstant/CampBx/Bitstamp/etc. and move those BTC to an altcoin exchange.