Hi Xarra, I use a Lowepro Rover Plus AW. In the camera compartment in bottom I have my gripped 50D and my 50-500mm (detached). In the top section I get in waterproof trousers and jacket, flask and lunchbox along with filters CF cards etc, batteries in the side pockets and it has a great tripod mound down the back of the bag.
At times when I dont take my flask and lunchbox I usually stick my 400D with lens fitted in the top section wrapped up in my waterproofs and a spare lens if needed in one of the interior pockets.
Very comfy bag to wear even when fully loaded with nice big padded waiststraps and a chest strap too. Happy to take any pics of it you may wish to see with or without kit inside.
On days when I want a smaller bag but still want my 50-500 along with me I use my Lowepro Fastpack 200 with lens detached in top section and camera bodies, flashes and spare lenses in bottom camera section. No tripod holder on this one though so it gets flung in it's case and over the shoulder.