If you have a large budget for this then I will third the McHale suggestion. Dan makes some stinkin huge packs if you want 'em big. I myself have a smaller McHale (Popcan that's probably 4,200 - 4,400 cu in). As far as I'm concerned his packs are hands down the most comfortable you can buy, and for the size/weight you are looking to carry that is an important thing to consider.

One comment on something Paul said. He said something along the lines of you can get a large McHale for around 3 lbs. My pack is about as stripped down as you can get in the lightest materials and it weighs slightly over 4 lbs, so I doubt something as large as what you are looking at is going to be under 5 lbs. If you want more details on my pack search the forums as I did a detailed writeup about it in a post sometime last year.