I second Within Temptation and After Forever, I don't really like the music but the singers are good and your g/f might like em.
Also, the latest Ayreon CD 01010101001 or whatever its called has a lot of good female singers (the one from After Forever included) and they sound amazing. Great music too. And theres Jorn.
someone played me a female-fronted band's CD in the past and I really dug it. I've tried to recall what it was for years. With some recent looking, I think it was Fear of God's "Within the Veil."
I have a demo tape from a "hardcore-ish" band called Ashes. It's got a low-budget production, but the girl has a very sultry, soothing voice. I'll probably be converting it to MP3 form soon - I'll PM you if I do.
I'm friends with everyone that was in this band - Mod Effect from Dallas. Unfortunately they broke up a few months ago, but 5 of their songs are up. Let me know if you like it, I have both their 1cd and the demos for their 2nd cd that never came out. Both produced by D. Braxton Henry (Drowning Pool and Big Iron). The singer is married to the founder and lead guitarist of Big Iron, which also is no longer around.
Anyways, check out the song Afterlife. It was their tribute to Dimebag. I'm sure you will like it.
I wouldn't really call it metal, but if you don't mind French lyrics, you could check out mypollux. Best female vocals I've heard thus far. Plus they have a song with guest vocals from Joe Duplantier of Gojira. Great song.