Unfortunately, the low-light graininess won't be solved by a new lens. The D90 was a great camera when it was released but it is "old" in regards to low-light capability. I owned one and loved it. It just doesn't have great low-light capability. That problem can only be solved by a new camera. Alternatively, if you aren't shooting in RAW, you should consider shooting in RAW and seeing what results you can get in post-processing regarding noise reduction. There are some very good de-noising tools out there. If you were to consider a "new" body, there are a lot of good Nikon bodies out there. The newest equivalent to your D90 is the D7200. You'd probably be quite happy with one of the updates to the D90 that is inbetween though, as you'll be able to pick up D7000 or D7100 much cheaper. From the reviews, the D7200 is a much better camera, but will be expensive new. I've heard the 5500 is also quite capable, but it is set up very differently than your D90 (i.e., button layouts etc.).
If you want more range and higher quality than the 18-105 lens, you have several options to consider, but it basically comes down to primes vs higher quality zooms or a combination of both. This is a hotly debated topic as you can see from the many, many threads on this forum.
Here are a couple of options for you to consider that presume you are keeping your 35 mm, with an attempt to keep the suggestions reasonable in terms of price and restricted to somewhat newer lenses. However, an increase in lens quality almost always comes at a price as I'm sure you understand. I don't have much experience with AF-D lenses and earlier, and others might be able to offer up some gems that are worth considering. Others will also definitely offer contradictory opinions. I encourage you to consider them all.
- set of primes: wide angle (24 or 28 mm f2.8D?), 50 mm f1.4D, 85 mm f1.8D + Tamron 70-300 VC Approximate price: $400 + $300 + $460 + $450
- you could also buy one of the above + the Tamron. Along with your 35 mm, it would give you pretty good focal length coverage and good quality at a reasonable price.
- Sigma 17-50 f2.8 DC (DX) OS + Tamron 70-300 VC Total approximate price: $470 + $450 - my fiancee owns the 17-50 and loves it.
You can, of course, get all these lenses cheaper if you buy them used.
I would discourage you from purchasing any super wide range zooms (i.e., 18-200, 18-300 etc). Many people find the image quality compromise acceptable given the convenience, but if you're looking to increase image quality beyond your kit lens, I don't think this is the way you want to go.
I suggest the Tamron repeatedly because it is highly regarded (most believe it's better than the Nikon version). Another viable option is the Nikon 55-300 DX VR. My father and fiancee own it and it is a decent quality option to get you to 300 mm at a reasonable ($400) price. I think you'd be better off spending the extra $50 to get the Tamron.
I don't do any product photography, so additional focal lengths may be of use to you. If you require a longer focal length prime for product or portrait photography, I might suggest a Sigma 105 f2.8 OS macro ($770). It is cheaper than the Nikon version and is nearly its equal. There are also good macro options by Tokina in a similar focal length range that are considered good longer length portrait lenses.
Sorry for the long response. Hopefully it provides some useful advice.
www.flickr.com/photos/sgtsalamander
