You might have started out with just a DSLR body and a single lens, but chances are, as time has gone on you have accumulated a lot more photography gear. So when people ask me what camera bag should I get, I usually answer, "you should actually get two bags." A solid photography tip is definitely have more than one camera bag. I have only two myself, and I could definitely use a third because neither of my bags can hold all my photography gear for a typical job in a single bag.
The Lowepro Flipside 200 has been a great bag. I can hold a lot in its very slim profile. As you can see it can hold one DSLR body, a 70-200mm f/2.8 lens, a second medium sized lens, a speedlight, and hidden at the very bottom of the bag in a small compartment is a 50mm lens. It also holds spare batteries, memory cards, camera cleaning equipment and a water bottle (very important!).
When I do not need to bring my long Nikkor 80-200mm f/2.8 lens, then I can often get away with just using my much smaller Lowepro Nova 160 AW bag. Despite its small size, I can still fit a DSLR body, medium sized lens, speedlight and a 50mm lens plus memory cards and extra batteries, and the all important water bottle.
Basically, the amount of gear dictates how many bags you need and what sizes. If you have a lot of gear and one big bag that can hold everything, I recommend getting a smaller bag for times you only need one lens with you. Likewise, if you just have one big bag and it makes you not want to go out shooting as much because it is too heavy to lug around all the time, a second smaller bag is a must have.
How many camera bags do you have?