Hi Julia,
The D90, although superceded now, certainly isn't a bad bit of kit and I found it to be pretty user friendly. When I first got it, I was a bit bamboozled by all the additional features though. When I picked up the up the E-510 again, it felt tiny and a bit sparse lol. The only thing I miss about the E-510 is the metering. I used to use some wonderful old manual focus glass lenses on it. The D90 doesn't have the ability to meter non D or G type lenses, which is a shame, as for me it was its only drawback.
The best thing to do is read reviews and the cameras' specifications and see what meets your requirements. Then go and hunt out those models and see what feels best in your hand. Going from the E-510, the D90 felt huge and a little cumbersome initially but, after a while you just get used to it and now it feels perfectly natural, so bear that in mind. Any new camera is going to feel a bit alien.
If you thought the D90 was heavy, you should try it with battery grip on the bottom and a solidly built lens lol. I had to buy a new camera strap, as the weight of the D90, grip and my Tokina 28-80 f/2.8 was REALLY hanging heavy on my neck.
You could splash out on the latest models, or you could sit tight for a bit and see what happens to the price of older models, giving you a bit more to lavish on lenses and accessories. I have to say though, the new Nikon D7000 does look bloody awesome, with its 16.2MP censor and 39 point AF system, plus the new exspeed II processing engine. My D90 looks a bit naff on paper compared to it
Tell you what.... buy the D7000, if you don't like it, I'll swap it for my D90. Can't say fairer than that!