I just went to the site you linked and now I want another one too! Tiffany's is great. I also love David Yurman. I do believe you can invest in gorgeous cocktail rings, especially if you wear them a lot. I also get jewelry from certain jewelers, and even if it's another $1200 or so, they are life long investments you can hand down and resize accordingly. Plus, there are rings that are waaaaaay more, but just see what you can afford realistically.
Have fun! I always throw on a cocktail ring before I head out.