Still very new to photography myself, was in your shoes not very long ago. I started with a Sony a200. Which isn't a bad camera either. I just recently acquired a Nikon D300 from a good friend of mine. Kinda like jumping from a Honda Civic to a Cadillac for me.
I can't really recommend a system or body as I havn't had much experience with different systems on the same level. (D90 vs a550 vs EOS 450D for example). But what I DO know is that whatever system you do go with, Cannon, Nikon, whatever, try to stick with it. Switching systems gets very expensive if you don't use the other system you have. A good lens on a cheap body won't use the full potential of that lens, but when you upgrade to a new body, that body will. Things like good lenses, filters, those things things move up very well. Someone once told me that lens technology really only significantly changes every 10 years or so, bodies change every year it seems. Dunno if its true, but I understand the point.
Also remember not to focus on equipment quite so much, because if your just starting you probably will at some point. As in, if I just had that lens, or filter, or software, I could really make a better photo. Get your basics down ever before you buy your next lens. Get a good tripod tho... And a UV filter to put on the front of that lens to protect it.
As far as being to advanced, the D90 I don't think really would be. But again, start with the basics,
Fundamentals of Photography by Tom Ang I thought was very good book on the technical foundations of photography, but don't stop there either.
The Creative Photographer by John Ingledew I thought was a great book on the artistic medium that is Photography. I'm sure there are others, and better ones. So pick up some literature like that too. The camera itself won't give you fantastic photos, thats up to you, so don't fall into that pitfall. and the market will try to do that. Also stay away from photo editing to start with, can't turn a bad photograph into a good one!
I don't think i'm wrong in any of that?