Weight/size of the camera would be important to me so I guess that rules out a full frame. I liked my dads canon 550d, but that is probably old technology now? What's this/next years equivalent?
With the Canons the models move up like so: xxxxd, xxxd, xxd, and then you have the 7d 5d and the 1d, (and I think they've just recently introduced a 6d).
Single digit models are higher up the tree, with the 1d being at the top (at least in monetary terms).
The 550d is a consumer/entry level model. There are the 1000d and 1100d below, which were introduced later on, but the xxxd series is the common one. I think this years would be the 650d or something. Newer models will have better low light capability, video functions and so on, but you could go way back (350d say) and still get excellent pictures.
xxd series are more prosumer market. Well built, robust, bigger and in my opinion have friendlier and more versatile controls. Not so much designed for simpletons.
xd series is moving into serious and expensive kit category. Do remember that the body is a small part of the puzzle. More money will be spent.