I didn't like the chick who replaced Katie Holme as Rachel Dawes. She didn't look good and her acting was weak too. Other than that the movie was really good. Ledger was awesome.
Maggie may not be as cutesy as Katie (although far sexier, especially compared to the new scientoloholmes) but her acting is normally top notch. Haven't seen this movie yet but have liked her in pretty much anything else she's been in. Of course the fact that you don't know her name might give me directional insight into your cinematic tastes in which case I should have probably ignored your post. 
I don't know many actors/actresses names. I usually recall actors/actresses from their roles. "Oh thats the guy from _ _ _ _ _." I looked up Maggie on imdb and I really haven't seen many of the movies she has been in and the ones I have I really don't recall her. I do remember her from "Secretary" a movie that I only watched half of because it was just too out there for me.
Adaptation, Happy Endings, Donnie Darko and Stranger Than Fiction were very good-to-great (not necessarily the best films ever but way above average) and regardless of the size of the roles she was very good in all of them. If you haven't seen them then watch them on or rent them from Netflix. Secretary was very good as well, not sure I get your "out there" comment. She was supposedly great in Sherrybaby as well but that hasn't reached the top of my Netflix queue as of yet.
I did like Holmes in The Gift for two obvious reasons.