Scribblah - You'll like the Lowpro bag. I have the slingshot 300 and it works well. You might be ok with the 200 model if you don't have many lens, or a longer lens you want mounted while backpacking. For Magic Kingdom I would go with the 100 or 200 model because of the physical size. Peterson book on Understanding Exposure is the best purchase for the money you could have made at this point in your photography endevor.
Listen to the Sony folks like Don and they will lead you into the best lens for the money.
Have fun and get some great photo's with that new A300.
Canon EOS 50D,7D and some lens and equipment.
Fuji F200 Exr