The D90 is a fantastic camera and in my view it will become a classic in years to come.
The 18 - 105mm lens is a very good lens and with a bit of PP editing you can get some really excellent result.
The 50mm prime will allow you to get better results on portaits with a shallower depth of field that a f1.4 or the more affordable f1.8 offers, even when stopped down to f2.8.
I took my D90 to Malta a few years ago with a Nikon 50mm f1.8, the 18-105mm kit lens and the 70-300mm VR. I took over 600 shots with the first two lenses and a few dozen with the 70 - 300mm.
You can pickup a Nikon 50mm f1.8 secondhand for about £40 - £55.
I only started to ues my 70 - 300mm VR in anger when I tried some wild life photography and sports stuff. However, for the sports stuff, I preferred the more expensive 70 - 200mm f2.8 VR as it's a faster lens. I realised I needed a much longer lens for wildlife so I invested in a sturdy tripod, a small bean bag and a Tokina 80 - 400mm ATX Pro lens. The 70 - 300mm VR then became redundant and only used to taking on holiday as it's lighter than the Tokina.
I love both my Sigma 8-16 and Tokina 11 - 16 lenses. I can't bring myself to selling the Nikon fit ones even though I use a 7D now. I tend to use the 8-16mm for arty shots where foreground subjects are very close-by but I have an interesting background or where I want to emphasise the subject matter or where I want to go really wide.
The Tokina 11-16mm is mostly used for regulation lanscapes.
If you're like me, you'll end up scouting ebay and the internet for great bargains.
If you weren't too fussed about the reach of the 18-105mm, the Sigma 17-70mm f2.8 - 4.0 OS HSM lens is fab as it almost bridges the gap of a Prime lens (35 or 50mm). considering you can pick it up for £250 - £300, it's a much sharper lens with better all-round optics. It's much faster than the 18-105 and has a faster AF.
Then you would be justified to invest in the 70 - 300mm VR. The Sigma 8-16 or the Tokina 11-16 could then be your wide lens option, if you need to go wider than the 17mm of the Sigma 17-70mm.
Nikon have a 24-120mm which comes in a value model and a constant f4. To be honest, I've tried them both and I couldn't justify spending £650 or more on the f4 model. I might not have any choice if I were on full frame, but as an APS-C user the standard 24-120mm VR model should be considered. Stick with the 18-105mm is more versatile in my view especially when you take into consideration the 1.5x APS-C crop factor.
I just wish Sigma would bring out a 17 - 125mm f2.8 - 4.0 OS HSM for a reasonable price, based on their 17-70mm
Your choices will ultimately depend on the type of photography you're interested in. Good luck.