Verified answer

Simply: D7000 and a fast/large/wide aperture lens

The sensor in the D7000 is excellent at controlling noise in low light / high ISO and a fast aperture lens is also a must

I would consider Nikon D7000 16.2MP DX-Format CMOS Digital SLR

*High Resolution 16.2 MP DX-format CMOS sensor

*Body only; lenses sold separately

*High Speed 6 frames per second continuous shooting up to 100 shots

*Breathtaking Full 1080p HD Movies with Full Time Autofocus

For that you'll mostly not need a better camera, but a better lens. Consider the 35mm/1.8G, 50mm/1.4G (not D, as that one won't autofocus on your camera!), or a fast zoom lens with a constant aperture of f/2.8.

If you wish to upgrade your camera body, then it should be between the D90 and the D7000; i see little reason to consider a D300s with the D7000 available, as the D7000 equals and even beats the D300s in very most aspects that are relevant to amateur photographers and costs less than it.

All of the upgrade candidates of the bodies have the advantage that they'd also work and autofocus with AF lenses, such as the 50mm/1.8D. All of them do better in low light than the D40, among them the D90 and D300s are equals in this regard, and the D7000 beats both of them.

I went from D40 to D90 myself too, and so i'm a bit biased towards the D90 as it does everything i want and much more than that - i drool at the specs of the D7000 but i realize that i don't need that camera (yet).

It's difficult to make a specific recommendation without knowing the budget, and what lenses you already own. But given that you listed a D300s as a candidate, i presume that you're willing to spend $1600 or so.

To me, that would then be the D90 with 50mm/1.4G and Tamron 17-50/2.8 VC - an awesome combination for low light, but somewhat lacking longer reach. Or the Tokina 50-135/2.8 instead of the Tamron, but then that'll lack in the wide angle department.