I'm travelling in South Africa at the moment, and carrying a Think Tank Airport Addicted Camera bag for storage of all my gear. It's great, but very heavy (+/- 50lb's). I've got two bodies, a 17-35, 28-70, 70-200, 300 f/2.8, a 28mm PC, 2 SB flash units, an SU-800 controller, plus some Honl snoots and flags, a digital sound recorder and mic, and all the batteries, chargers, cords, and power adapters for the above. Carrying it through the airport is a tough haul, so unless you're really strong, get one of the wheeled bags.
For walking around, I have a belt pack that has one of the large pouches, big enough to hold the 70-200, the pouch for the SB flash, and the speed changer bag, which I ripped all the dividers out of for extra room. With the two cameras, one lens is always in the pouch, the other two mounted on cameras. I have an old Domke pouch for the recorder, plus anything else I might want to to take along. I find the Domke canvas belt more comfortable than the Think Tank padded one, as it kept slipping off my hips. For casually walking around, I tend to sling the belt pack over my shoulders like a shoulder bag, and for more intense stuff, I wear it around my waist.
I've also got a Airport ChecK In computer bag that I can't say enough about.
If you want a pure over the shoulder bag, the large Billingham is the very expensive ticket. That's what I use for my wedding work, but again, you need to very strong to carry that kind of an unbalanced load on one shoulder for the whole day. The Domke's were designed before digital cameras came along, and just don't hold those big cameras and lenses very well.
If you want to check out some snaps from my current African trip (I'm writing from Durban, South Africa),
check out my blog. David Buzzard