The D90 is a great body (I have one and love it), but it is nearly twice as expensive as what you can find a used D80 for so just make sure it's worth it to you. I shoot every single day, so it was worth it for me, with the main advantages being:
- Better high ISO performance
- Bigger, brighter, better LCD screen -- this thing is fantastic
- Improved matrix metering
Other smaller things that add up for me also are:
- Live view (I don't use it often, but on occasion it's *really* handy)
- Built-in anti-dust feature
And of course there's video if you care, but I still haven't used that except for shooting one test vid just to see how it works.
Those changes made it worth the extra $400 and change over a D80 for me.