In formal emails, I have seen and used 'Dear Sir or Madam'. I'm not sure what the American approach is, but RonBee willbe along soon tohelp, I'm sure. 'Hello' strikes me as a bit informal.
"Dear Sir" would be fine with me. Also, as stated, use "Dear Sir or Madam" if you don't know who the contact person is. I do think MNY probably knows more about business emails than I do.
In informal emails it is typical to have no salutation. (The subject line serves that function.) I don't know what the rules for formal business emails are, but I don't think the salutation is necessarily all that important.