The logical upgrade would be to the D90, but it is about to be replaced. The announcement should be made in less than one month's time. No one outside of Nikon actually knows the details, but it is pretty easy to guess some of them. It should have at least a 12MP sensor, with the possibility of as much as 16MP.
Expect much better low-light performance, probably with ISO3200 producing impressive image quality with little noise as long as the image is not underexposed. It should show up with at least ISO12800 or even ISO25,600 available with boost. However, noise may be profound above ISO6400. Expect full HD movies.
The superb 3" full VGA monitor will carry over - almost worth the upgrade price itself - which can be used with LiveView. I have had this monitor on both a D300 and my present D700 and it is incredible.
What will be most noticeable is that the camera will be much more mature. I went from a D200 to a D300 and while they did not look dramatically different comparing specifications, in the field, the D300 simply did everything better - much better. Cameras arriving in the year 2010 are vastly improved over those of 2005. The big breakthrough came in 2007 when the D3/D300 were announced. The D90 which came along a year later, equaled the image quality of the D300, but at a substantially lower price.
The price should be about what the current D90 sells for, though I expect that remaining stock may be discounted until the channel has been cleared. Buying the successor will put you at the forefront of current camera technology on a consumer level, and should be a camera that will provide great images and equally great satisfaction for a number of years.
Why not just buy a D90? Simply that it is at the end of its life and you are buying old technology. Thus the temptation will be to replace it a couple of years earlier. You may save a bit of money initially if the D90 is actually being discounted, but the savings will be illusory.
Cameras are in fact digital image acquisition devices, and they follow Gordon Moore's Law—as do all digital devices. When producing new cameras, the makers don't just move a bit of trim about and slap on a "New and Improved" sticker. Each generation produces significant advances over the previous one. Since the release is apparently imminent, it would be worth waiting a month or two until the new camera has been reviewed and is on the store shelves.
What to say over and above larrys response ... Erm yeah, I think you would be wise to wait and see what comes after the d90... There will be a time where both will be in the big camera shops ... So when that happens pop in and weigh the odds ... The main problem is the launch price of cameras is so high compared to that of what it settles to after a few months so if you decide on the new camera you might have to wait a bit longer before it's a sencible purchase