No need to take offense if a girl calls you dude...
It shows she is
a) comfortable around you
b) in touch with her masculine side
I read somewhere that girls calling guys "dude" is the equalivant of guys calling girls "babe" , not sure how accurate that statement is...but go with it.
Ive been called "dude" several times by girls and I think its more because they are comfortable talking to me...
I would , however, in your case and in general rather have the girl say "hey babe" or "hey sexy" . I get that alot , but sometimes a girl may say "dude" try not to over analyze it... rather analyze the other things about her ,
Her interest level for starters?