http://english.stackexchange.com/questions/5265/an-sql-serve...
http://english.stackexchange.com/questions/1016/do-you-use-a...
That said... As a native English speaker, I can infer your pronunciation from which of the options of "a" and "an" you decide to use.
FWIW, "an" is appropriate for any word starting with a vowel sound (whether or not it actually starts with a vowel) and inappropriate for any word that doesn't. "An historic" drives me nuts, but I'm one of those jerks. (My mother was an English teacher - I never stood an chance!)