I think you've received lots of good ideas here, Astrid And I'm going to throw a few more ideas at you.
Although I know a "dressy" dress is a important wardrobe item for many women--and you've got some great options shown here--I've never found that the "dressy" dress made sense for ME. I've balked at the prices, to say nothing of the need for investing in a host of support pieces like "dressy" shoes, bags, and jewelry which sit in my closet, waiting for the right occasion. BUT, Fabbers are right in thinking, as adult women, we need to be able to have something in our closet for special occasions.
My suggestion would be to find a basic, solid black bottom piece--trousers or skirt-- that works with your shoes. Ideally the bottom piece is in a fine wool or other "luxe fabric", and is a flattering but simple style. This bottom piece and your shoes become your foundation. Once you have this foundation in place, you add an upper piece--jacket, sweater, soft shirt, colour, sparkle, jewelry--whatever suits you, the occasion, and your budget. The upper piece is always your statement. Over the years you adapt your "dressy" outfit by finding additional statement uppers, dressier shoes, special toppers--whatever fits your needs and budget.
I've used this formula for years because it allows me to adjust the degree of formality without requiring a closet of "dressy" clothes. The foundation and the upper pieces can be mixed with other pieces in your wardrobe. Getting dressed for a "dressy" occasional becomes a question of determining the right upper piece. If your events start veering towards semi-formal, search for a dressier shoe and bag to add to the capsule. Tweak the formula whenever events or your style changes.