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I know Don't shoot a Nikon, but I have a 70-200mm F2.8 canon, and I used to have the 70-200mm F4.
Originally I bought the F4 thinking 2.8 wouldn't be necessary. (and to an extent it isn't) but after giving in and getting the 2.8, which has basically the same optical quality, but much faster, it was worth it to me.
I wouldn't shoot It wide open, you'll have trouble getting your subject in focus, but rather, shoot at about a 3.2 and it will get rid of some of the natural vignetting and give you a little more wiggle room with the focus. And still be excellent at night.
How high are you going on the ISO? 1600 should be ok (i haven't shot a D90) but if you're wanting better ISO, consider picking up a used D700 and just cranking the ISO. Though you will lose focal length with the full frame.
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