Esancho - it always seems like the body is the place to upgrade, but the D90 is a solid platform, and there were only three things I ever complained about with mine.
ISO performance - I love shooting in low light, but you have to go to the D3s to get 2 stops of better ISO performance, where 1.8 glass will give you almost three stops! And the glass will work on your next body!
Handling - Now that I have a D300s, I really prefer it to the more menu-driven handling of the D90/D7000 and even the new D600 (which I think should have been called a D8000, but whatever). But the D90 handles very well.
AF - the AF is a little limiting for action and gives you less points to work with than a D7000/D600 (39 points) or a D300s/D700/D800 (51 points). Honestly, this is often as much about how you shoot as anything. The D90 AF system is pretty darn good.
I upgraded nearly all my glass to f/2.8 zooms and f/1.8 primes, and it made me fall back in love with my D90.
Get a 35mm f/1.8G to start, and look how amazing the photos are. My used Tamron 28-75 f/2.8 (the older non-built-in-motor model, called "non-BIM") was a steal at $325 and is stuck on the front of my camera a majority of the time.
Buy yourself some time with glass, and I bet you'll be really happy you did.
Also, you might want to check out Scott Kelby's "Digital Photography" series (especially the first two) and Bryan Peterson's "Understanding Exposure" (in that order). Those books were a massive help for me.