I'll second the earlier recommendation for a 50mm prime. I've got a Nikon 50mm f/1.8, cost me maybe $100, and I take about half of my pictures with it. It's the best lens you'll ever buy and also the cheapest. Similarly amazing is the 105mm macro lens, if you're into that sort of thing. (The new one has vibration reduction; I have the older one.) The 50mm prime makes for a fantastic portrait lens and it's great in low light. It's also stunningly sharp. You'll compare the results with your 18-200 and it will make you cry (or, make you go out and buy other prime lenses).

I'll second your choice to go with the Really Right Stuff gear. I've got the quick-release clamp and the L-bracket, and they're absolutely fantastic. My brother-in-law, who does automotive engineering for a living (he works for Chrysler), was really impressed with the build quality and metal working when my wife got me that stuff for Christmas.

As to the Sigma 30mm vs. the Nikon 35mm, and any other lens purchases, you might want to wait. The rumor mill claims Nikon has some new prime lenses in the pipeline which may (or may not) be announced at the big PMA show in February. I agree that DPReview is too painful to bear. I have Nikon Watch on my RSS aggregator. That's a good place to keep up with Nikon tech announcements and rumors with fairly low noise.

About the only other gadget I'd recommend getting yourself is a decent white balance card. I've got a WhiBal card set. These are neutral grey plastic cards. When you're in a tricky light situation (florescent, incadescent, daylight, mixed together), you can take a reference photo with this and then, using something like Adobe Camera Raw, it's a single click to get the white balance correct, which you can then apply to all your other pictures in a single go. Well worth it. (I used to be insensitive to all these different variations on white light, but now the variation drives me batty.)

As to what to go and do with your camera, you clearly already know a good bit about photography, so the thing to do is to go out and do it! The main thing that's going to be truly new and different is the sheer joy of being able to shoot at zero marginal cost. You never take just one shot any more. You shoot a million variations on the shot. Why not! The screen, particularly with the color histograms, is fantastically helpful at helping you get ideal exposures and/or compensating when the exposure meter is off.