I can only speak of what I've had experience of using, so keep that in mind.
I've found that a good reasonably priced option for portraiture can be the Nikon 85mm F1.8 D AF. I use the F1.4, but the F1.8 can do a really great job.
Another prime I've used for portraiture is the Voigtlander 58/1.4 - and I found it has the bonus of being a little more easy to work with when space is at a premium indoors.
I'm kind of conservative with my lens choices, so I stay with what I know and it's rare that I'll try anything new or different. When I've found something that works I stick with it. However there are two other lenses which might fit your budget that I'm planning to spend some time with (again for portraiture) are the the Nikon 60mm f/2.8G and the Tamron SP 90mm F/2.8
I'm sure you'll receive plenty of other suggestions. One other option if you've not tried already might be to have a look at somewhere such as pixel-peeper.com or even flickr.com and see what sort of work is being produced with various combos of the D90 and different lenses. It might give you some insight and then you can concentrate on what works for you. In any case, good luck in finding exactly what you're looking for.