Office Fashion: Dresses

Having been in my black suit "uniform" for a month now, I am ready for something different. Something a little more girlier and fun, but still suitable.
Rummaging through my closet, I came up with my Ralph Lauren shirt dress this summer. I've always thought it was a bit formal for everyday casual wear (though I wear it anyway), but now thinking about it for work, it just seems too casual. Kind of like I'm going to spend a day at the country club instead of a serious day at work. Although it IS a shirt dress, complete with a collar and all.
Going a bit deeper, I found this black silk and chiffon dress. I actually thought about wearing it to dinner and then to a club the other night. But decided that it was too formal for a club and it looked like it was for work. Then again, the chiffon and the silkiness of the dress makes it seem too dressy for work.....Next in my wardrobe comes my Diane von Furstenburg wrap dress. I ended up wearing this to dinner and club the other night. This is classic. It is supposedly perfect for all occasions. But then my only worry is (and yes, I know, I worry too much, people probably don't even care), is this too nice for a summer intern?

So ladies, what do you think of the dress in the workplace? Is there such a thing as too formal for work? And what does it convey about our professionalism?

Image Source: Net-a-Porter, Ralph Lauren