50D is a step or two up from a beginner's camera. It's usually best to start out with something more basic and learn the basics on that, make full use of it before stepping up to a better model (which will be updated by the time you are ready...ie the 60D).
The Tamron you sold off is a fantastic lens...it's a shame you didn't hang onto it. Going to the Canon EF-S 17-55 F2.8 IS USM is (in my humble opinion) a waste of money at this point in time, until you master the basics such as the questions you are asking about depth of field etc.
My theory: Buy a basic camera, and learn, learn, learn. Eventually your needs will exceed what the camera offers, and that's when you buy the next model up. Same with lenses. Otherwise you're just throwing money away for something you'll never make full use of (and will not make your photographs any better).