I think that you can use either "of" or "about". However there are times when neither "of" or "about" is used: Last night I dreamt/dreamed that I was in a car accident". "I dreamt/dreamed I fell out of the window".
Well, I don't know if it's a rule or not but Hormiguita has a point.
The trouble might be with dreams. There are actually dreams while we sleep, but also dreams that are hopes, aspirations. "Dreams of stardom" Those dreams, always formulated with "of," are happy. If someone were to "dream of torture" it would only be in that he would enjoy it.
I think with actual dreams it's generally more current, at least in the US, to use 'about,' though 'of' is certainly not wrong.
There's also a difference in the focus. "I dreamt about you last night" means you were the focus of my dream. "I dreamt of fields of mimosa" just means they were an image in my dream.
When in doubt, as joylolade suggests, avoid it altogether, and write "I dreamt I was hit by a car."
I've always enjoyed the choice in English between "dreamed" and "dreamt" in that they're pronounced so differently and so the choice gets to be mainly a musical one.