I used the Sony A300 for 2 years, and found it to be an excellent overall camera. It shares the same sensor as the one in your camera - the only differences are the design of the camera body, and that mine had the live view. I've taken tens of thousands of photos with that camera, including some hired shoots and have sold and published photos from it - it is plenty capable and will give you lots of room to learn and grow. Now it's just a matter of getting some nice lenses as you grow past the kit lenses and want more. Get used to the camera with the kit lens, and when you feel you've really gotten the hang of it and are confident with it, then start considering moving up to some better lenses. That's the nice thing about DSLRs - even with entry-level models, you can stick with the camera body for a while, and just worry about upgrading lenses. And when you decide to upgrade the body, the lenses can carry over.
I went to a Sony A550 from my A300, and have been enjoying this model too - no complaints from the Sony lineup of cameras and lenses for me. Don't worry about whether you made the right decision on brand - Canon, Sony, Nikon, Pentax, Olympus - you aren't going to go wrong with a DSLR purchase nowadays - they're all excellent. And even entry-level DSLRs are more capable than well over half of the folks using them...so there's plenty of room to grow!
Enjoy!!
