I'll probably get myself in trouble for saying this, but (while I slip into some asbestos underwear) that 20mm prime is not considered a great lens, although it might exceed your 18-200 at 20mm. Either way you're duplicating existing functionality by replacing an ok lens with an ok lens. With your kit, that's not where I'd recommend you put your money.
I don't know what you're paying but in that price range, ultra-wide options like Sigma 10-20mm f/3.5, Tokina 11-16mm f/2.8, and Sigma 8-16mm f/4.5-5.6 will open up new worlds to your camera.
Or if you're looking for a faster lens to do subject isolation, AF-S lenses 28mm f/1.8G, 35mm f/1.8DX, 50mm f/1.8G, 85mm f/1.8G will offer more interesting options, depending on what subject matter interests you most.