Sigma have some pretty good lenses. Your Nikon kit lens should be pretty good. If you not taking professional photos, I really don't see how you could notice a difference. A big question is how much you want to spend, and what type of photos you are trying to take?
If you want a lens for family photos and a zoom lens all in one, they can get pretty pricy. Unless you want to stand far back, anything higher than 50mm is no good for point and shoot.
Have you tried different setting on you camera to help with the picture quality? What is wrong with the pictures. I'm no professional, but have owned a DSLR for a few years