I use a Tamrac Expedition 7. Handles everything from my SLR systems up to 4x5 (never tried to jam a 5x7 in there). A 2x3 Graphic will actually fit turned vertically. I've had it packed to about 30 lbs, at which point I find the harness isn't providing as much support as it needs. I've done a bunch of long hikes with it at around 25 lbs though. There's a slightly larger version but you need to decide what you really need and limit it.

Dan