Congrats on your new camera! Really liking your pics so far.
As for your lens dilemma.... the price that you quoted for the 18-200 sigma lens ($200), is that for the sigma lens with image stabilization (equivalent to Nikon's VR)? The price seems quite low for a stabilized lens. If it was that cheap, I would've certainly opted for that instead of carrying multiple lenses. I had the Nikon 55-200 (VR), pretty good lens, but sold it and got the Nikon 55-300 (VR). So now I have the 18-55 (VR), 55-300 (VR) & 35mm (f1.8). If you are considering a zoom lens, I would strongly encourage you to get one that has image stabilization (VR). Although you can use a tripod to counter camera shake at the tele end of a zoom lens, there will be times when you can't use a tripod and the image stabilization is therefore a must in my opinion.