What you want is the lowepro pro trekker 3,4 or 600. I have used backpacks from most major manufacturers and this is the first bag that has stood the abuse I throw at it. No ripped seams, tears, broken zippers or anything. Yes it is expensive but it is worth it. Here is an example of how much you can fit in the 400.
Nikon 70-200 2.8
Nikon 35mm 1.8
Nikon 60mm 2.8
Hasselblad with digital back as well as film back
sekonic 758dr meter
full cokin filter system
proshell jacket and pants
13 macbook pro
6 batteries, 8 compact flash cards and some sd cards
thats just off my head not including cable releases and things like that.
You want this bag. Trust me.