Well, Heavy metal with female singer is not the new thing. Already in the 80s there were such artists like Lita Ford, Doro, Girlschool and many more.
From my point of view 'the new wave' of female singer heavy metal was originated by Nightwish which started already in 1996. Opera heavy or not, Nightwish is a great live band and my personal favourite Nightwish album is 'Century Child'.
Within Temptation played here yesterday in the Ruisrock Festival, but I left the festival area before that...