I have bought and sold several bags, and I started thinking that all I needed was one bag to hold all my gear. I ended up with several bags, each has its purpose. this it what I have left and use:
Small crumpler with easy access, no zipper:
1 body with one lens (up to 24-70) and a 580 EXII
Spare battery, remote, memory in small pocket
Kata H-16:
1 body with 1 lens, up to 70-200 2.8, flash on one side, another lens on the other side.
Domke F-802
2 bodys with lenses: 1 body with 24-70 attached, 1 body with 70-200 attached
Speedlight in front pocket wit remote, memory, battery etc.
Lowepro Flipside 300:
1 body with lens, 1 extra lens, 1 speedlight, filters GND, ND etc
Think Tank Streetwalker HD:
Usally just for storage.
I started with the biggest bags, buying smaller and smaller. If I should have bought my first bag now it would have been the small Crumpler bag or Domke F-802. And then bought a backpack like the Flipside 300.