Thank you everyone, I'm going to keep my D90. I'm going to just put better glass on it.
I know, I'm late to the party. Missed this thread somehow, but I wanted to throw a couple of comments in since I do currently own a D90 and a D7000.
I love the D90. It is an excellent body with a lot of features that will allow you to grow as a photographer without breaking the bank. It has several things that the D5xxx series lacks, and is a far better body than the D3xxx series.
That said, the D7000 is a BETTER body in many respects. Magnesium frame with weather sealing, better sensor performance, better autofocus. Low-light is vastly improved over the D90, and I get about 2 stops better low-light performance with the D7000 than the D90. Even though the difference in resolution isn't huge (16mp for the D7000 vs. 12mp for the D90) I find that I can get away with significantly larger crops on the D7000 without losing too much resolution on the subject. Since I shoot birds a lot that's important to me. The control layout between the two is virtually identical. Pick up a D7000 after shooting a D90 for a while and your fingers automatically go to exactly the right places.
I did "Upgrade" from the D90 to the D7000 and it was worth it to me. The difference in performance between the two has been worth the cost from my point of view. I still use my D90 and I have no intention of selling it, but facts are facts: The D7000 is a superior body.
All that said, I respect your decision to improve your lenses. Going that route will position you with better glass when you do decide to upgrade to a better body. For now, unless you are finding limitations with the D90 (I was, I needed more resolution and better low-light) there is no reason to upgrade. There is absolutely nothing wrong with the D90 as it is a jewel of a body and, in my opinion, much preferred over the D5xxx series because of its features. Stick with it and enjoy it. When it's time for another body you'll know and won't have to ask.