Here are some thoughts on camera bags from a guy who has at last count 19 bags and cases for my cameras and more for my other photo gear (tripods, lights, stands and modifiers). I break it down into two categories: 1) a bag/case to get my gear to where I'm going to use it and 2) a bag to work out of once I'm on location.
Beginners often get a bag that fits their camera and one lens with room for film/memory cards and maybe a spare battery. Then they find themselves walking around with the camera and lens on their neck/shoulder with an essentially empty camera bag just dangling there for most of the day. Or they keep the camera in the bag and they keep digging it out, missing the moment they really wanted to capture, and then putting it back in the bag where it is useless. Thus I will often take two bags with me when I go out, one to get all my stuff there and then a waist or belt bag to keep the extras in while the camera in on my shoulder ready to be used. The more stuff I need the bigger the bags are but never more on me than I actually need.
I have two LowePro bags and although I like their Computrekker backpack I dont really like their bags. I'm a big fan of the Domke bags and some of the ones from ThinkTankPhoto. The big problem that beginners have with bags is that they think that they need a bag with a lot of padding which makes no sense. If you are carrying your gear padding makes the bag bulky and unless you are going to be banging it on things your gear won't be damaged. If your gear is going to be banged around you need a hard case which wont be on your shoulder at all.
If you need to carry any more than about 5 pounds of gear for any length of time get a backpack instead of a shoulder bag as its much more comfortable and wont hurt your shoulder and back. Whatever you get make sure that its minimally padded, has easy to use with one hand hardware and the internals allow for customization of the internal compartments to fit your gear. That said one of my favorite discreet shoulder bags for walking around is an Army surplus gas mask bag. It has a nice couple of internal pouches, will fit a flash, memory card wallet, spare lens, a snack and cost me about $10.