>As Tom said, you might want to consider spending the $1200 on >glass rather than on a new body unless you need or want a >specific D7000 feature. The D90 is an excellent camera. >$1200 comes close to getting you a used 70-200mm f/2.8 or a >brand new Tokina 11-16 f/2.8 paired with a fast prime. The >D7000 does work extremely well at high ISO settings, but fast >glass wins over high ISO.
F/2.8 zooms are not a huge advantage for low light shooting in my experience (I own the 17-55/2.8, 70-200/2.8, Tokina 11-16/2.8 and Sigma 150/2.8). It is more of an advantage at the telephoto end of the range where the kit zooms are closer to 2 stops slower.
Compared to say the 18-55 (3.5-5.6) and 55-200 (4-5.6) you are talk about a 2/3rds to 2 stop advantage depending on where you are in the range.
As I said the d7000 is good to ISO 3200 whereas the d90 realistically is only good to 800. That is a 2 stop advantage at any point in the range.
A fast 1.4 prime is another story, now you are talking about a real low light advantage. My Sigma 50 is my go to for this purpose.
Cost wise, if one were shooting in low light frequently, being able to shoot at higher ISOs consistently would beat fast glass any day as the cost of getting fast glass for a wide focal range is much greater.
Example: D7k = $1200, 2 stop advantage throughout the range, which is still a few hundred dollars less than the 17-55 and much cheaper than the 70-200.
Additionally, the d7k focuses better than the d90 in low light in my experience.