I have not used Valspar, but have used many other brands. You can certainly use this over your existing paint. Eggshell is best, but if you have flat already on your walls, you will just need to do a couple of extra coats. You will need at least 2, anyway. I don't know what your base color is now, but for best results, use a dark base coat. If you are unsure, do a small sample before you tackle the entire wall (you can always paint over).
It is a very effective technique, and will definately add drama. I did a small powder room, sans window, with a gold metallic on walls (using a dark brown base) and a silver metallic on the ceiling (with a deep pewter base). Simply brush the metallic paint over your base, working the room several times. It's OK to tape the mouldings.
I will try to add a picture...but good luck with your project!

I figured out how to post pics. Hereis a small powder room.

painter girl , that just looks wonderful, and I love the way you did the mirror to match. I am leaning toward a pale and darker violet and, funny thing, I will be putting a white and crystal small chandelier in this bath as well. Thanks. What a good photo.