The risk of FAS is increased if you consume alcohol on a chronic basis. I don't know the exact numbers but for women I'm pretty sure it's more than 4 standard drinks a day for most days of the week (the definitions change often but I think it's something like that). One drunken episode *shouldn't* harm your baby, provided that, at all other times, you've been drinking very little (if at all).
And just so you know, the recommended intake of alcohol for pregnant women is NONE. So as long as you haven't had any alcohol since you found out you were pregnant, and continue to be alcohol-free until the baby is born, he/she, and you, should be fine. =)