Actually, when you get away from Marvel and DC there are tons of comics out there that would appeal to women as much as they do to men, mostly by women. It's not that hard to pick something. Check out "Friends of Lulu" for more information:
http://friendsoflulu.wordpress.com/
If your girlfriend is cool you might give her Colleen Coover's "Small Favors." It's porno, but it's "girlie" porno, very lighthearted and life-positive. Also very well-drawn, Coover has a great talent and has done other work like "Bannana Sunday" and some Marvel short stories but it's more along the lines of kid's stuff.
If your girlfriend is cool enough for "Small Favors," she might also enjoy the raunchy "The Pro," written by Garth Ennis and drawn by Amanda Conner. Best Superhero prostitute story ever.
Alison Bechdel's "Fun Home" is also a volume I'd highly recommend. She has wonderful linework and it's a great female coming of age story and exploration of her relationship with her dad.
"Persepolis" is also a beautiful book, told from the perspective of a woman who grew up in revolutionary Iran.
Your girlfriend would have to have a heart of stone not to be touched by "Barry Ween: Boy Genius," the work that justifies Judd Winnick's existence. It's about the smartest and, in some ways, saddest boy on earth and it is also fall on the floor funny.
I'd also recommend giving her a collection of Donald Duck or Uncle Scrooge comics. Those are much more relatable than superheroes to most people. "The Life and Times of Scrooge McDuck" is a very good volume.
Oh, and I have to recommend one of the best comics I've ever read. Vertigo's "Enigma" is almost a superhero story, but it also deals with sexual identity. "Preacher" is another good Vertigo series to try, it's gory and gross but also unabashedly romantic.
If you want to try some superhero stuff, don't screw around with the latest just-okay Spider-Man effort. Go for the good stuff, try "Batman: Year One,"
"Daredevil: Born Again," "Watchmen," "Golden Age" and "Kingdom Come"
