I have this ... http://accessories.us.dell.com/sna/product...mp;sku=310-6736
I can tote 2 14" laptops with extra batteries and power cables, 3 avg size text books and other stuff fine.
When I go on short trips, I can cram 2 days worth of clothes, laptop, hygiene utilities and what not, camera, and depending on my form of travel...usually my concealed carry with 2 extra mags.
Those bags get heavy fast. Ive had that bag for about a year and its constantly crammed with my stuff...its holding up well and I use it daily.