What would be a good DSLR camera?

I’m going into my senior year of high school and for college i’m thinking about majoring in Marketing/Graphic Design. I have CS3, Gimp 2.6 and mess around with those and do layouts and such. Then last summer I wanted to get a good camera I could really use and learn a photo side of things. So I got a Nikon Coolpix P-80 with a Fish-eye Lens, this ran me around $300 (refurbished.) Now after using it for a year I’m finding it limiting me, the focus is really annoying and there is just little problems I don’t like with it. One of the big ones is not being able to get lens’ for it.

My question comes down to whats an affordable DSLR camera but still shoots good quality shots and is a good camera all around? I’m looking at Nikon D500-5000 stuff. But i’ll take any suggestions.
Keep the price around 500-700$?
Here’s some of my work with the Coolpix if anyone cares.

http://www.flickr.com/photos/wandeldesign/

Also I like shooting a lot of concerts.

I would recommend Nikon D5000 12.3 MP DX Digital SLR Camera
# D-Movie Mode with sound; record 720p HD movie clips
# Live View with contrast-detect AF, face detection and subject tracking
# Image sensor cleaning (sensor shake)
# 11 AF points (with 3D tracking)
# IS0 200-3200 range (100-6400 expanded)
# 4 frames per second continuous shooting

http://www.amazon.com/gp/product/B00267S7TQ?ie=UTF8&tag=bestdeals-y-20&linkCode=as2&camp=1789&creative=390957&creativeASIN=B00267S7TQ

you can read the pros and cons of camera here

http://www.best-discount-deals.com/1_4_buy-and-buy-compare-prices-Nikon-D5000-12-3-MP-DX-Digital-SLR-Camera-with-18-55mm-_100616448_018208254545_0.aspx

The Northbound non-stop train refurbished stock EMU at Tai Po Market Station

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Introduction to the Nikon D5000, Vol. 2: Advanced Topics Training DVD $29.99 Take great pictures with your Nikon D5000! Push your creativity to the limits; create photographic images that up until now have existed only in your imagination! This guide takes you through Canon’s advanced features, and explains what they are, how they work, and how they can perform in real-world situations. Custom Settings: The D5000 is endlessly configurable. We go into detail about the sett…