Are you looking for a true bag or a backpack? Backpacks are much easier to carry, but I haven't found one yet that I can pack like my Lowepro shoulder bag. You want to get it bigger than you think you need so you'll have room to grow, or at least room to operate. I bought a Lowepro Nova 200AW when I bought my first 5DMKII and all I had was a 24-105 and a 580EXII. I bought a smaller bag originally, but it was too small even for just that. I have a few backpacks, but it's still my favorite for carrying everything out of the house. It went from a body, two lenses and a flash(and accessories) to being able to handle a body, flash, seven lenses(including a 70-200 f/2.8 v2), 1.4ex and a LOT of accessories.
It's better to have a little too much room than not enough.