Well take Amy Adams for example no one knew who she was until Enchanted came out.

I don't really care how known they are, but usually I'd go for voice actors over famous actors just because it can be a little distracting watching an animation voiced by a celebrity you are very familiar with.
Really though, whoever acts best and has a voice that most suits the character. I think a lot of the time you just know when a voice fits, like Cinderella's voice is perfect for her, but would sound wrong for Ariel.

I actually did know Amy Adams, LisaForde, just because she was in an episode of Buffy the vampire slayer, lol.

Only if they work with the character.
I like to realise half way through a film that a character is voiced by a celebrety I know.

Just whoever does it best. I kind of get annoyed at movies that have nothing but huge celebreties acting (like "Horton Hears a Who" or "Bee Movie" to name recent ones), but you can't deny that sometimes a movie wouldn't be the same with out that big, iconic voice (like "Aladdin" had Robin Williams).