Most important considerations when buying a camera, for me?
1) Lenses available - Canon, Nikon, Leica M-mount, then all the nonOEMs like Sigma etc...
2) Sensor size - I'm a full-frame man all the way, APS-C is acceptable but you shouldn't really get anything smaller than APS-C in a DSLR/prosumer camera.
3) Pixel pitch - how far away physically the individual pixels on the sensor are from each other. Denser is better (generally).
4) High ISO performance - important for low-light images, don't like using the flash.
5) Ergonomics.
6) Feature set.
and then after that Megapixels (though it's somewhat tied into pixel pitch/density).
EOS 550D is a very good starter camera, I learned a lot with my 400D ages ago.
...incidentally, I once dated the niece of the guy who invented the digital camera. No sh*t. He was just awarded the presidential medal of technology in the US last year...