Quite a bit actually but it will all fit into a medium size laptop bag. You do not need a Tumi monstrosity. This may have been partially covered by the numerous laptop/school bag discussions, but on average quite a bit.
1. Laptop
2. Charger
3. At least 1 book but most likely 2
4. Notes
5. Case files/print outs
6. iPad
7. WSJ
What happens most of the time is that you start living out of your bag - you don't want to repack every day/night as you will inevitably forget to bring something it after a midnight study session the night before. Also, the impromptu meetings and free time opportunities often present themselves. This is the part that gets you really. You won't be as bad as a law school student (which often use a wheeled carry on bag, I am serious) who haul case files and dictionaries and other books around.
Many schools will have lockers where you can store stuff but I never found it useful. I preferred to keep all my crap on me.