Americans are expected to look as if they are highly competent, successful, and take their job seriously. Attention to detail regarding hair, for example, conveys competence. Quality clothing and jewellery conveys success. As I mentioned in my previous post about my first impression of BBC newsreaders, looking like you haven't combed your hair makes you look unreliable and they'll have trouble taking you seriously. There have been times I've said, "Dear Lord! She looks like she got drunk in a Top Shop, passed out trying on clothes, and then crawled into work wearing whatever she woke up in." I'm joking but the impression is real: the image of her getting drunk and then staggering into work in a state has been put in my head.
It's definitely important that your hair is perfectly tidy. Having a manicure doesn't mean fancy painted talons - I think if they're manicured like a man's nails (that is filed, perfect cuticles, immaculately clean, perhaps clear polish) they'll be alright. Certainly better that than to have a chip on the polish, anyway.
I was just looking at the websites for Ann Taylor, J. Crew, and Brooks Brothers. All three of them are perfect for the sort of clothes people wear in offices, although I guess high-level management may shop somewhere a bit more expensive. However, the models are misleading with their style (particularly their college girl hair). That's down to the stylists and the clothes in real life translate to something more conservative.
If you read that Oxygen link and follow through to some of the other related links at the bottom, there are a lot of good tips, particularly on dressing with authority. I noticed she made a particular point about wearing heels vs wearing flats. You're pretty much expected to wear heels, although not stilettos. One to three inches is fine.
I don't think NOT wearing big diamonds is a terrible thing. It's more important that you don't wear anything wrong. So, sins of commission far worse than sins of omission.