I was in Yosemite in 2004-05 (It's a really long walk to campus, so on the plus side: good excercise ; on the negative side: dont forget anything you need for class)
Make sure you have:
a mini-refridgerator (storing all of what your lovely + dollars can buy)
usually one roomate brings the TV and the other the fridge
(as a cool trick if you bring a big TV stick inside one of the closets it makes an awesome entertainment system.)
My roomate brought an Xbox, and he wasted his life till 4 am on it and got on academic probation. Likewise a lot of nerds fail out of cal poly due to gaming addiction.
Of course you should bring a computer (laptop or desktop doesnt matter)
The chair idea is also a good one, but basic chairs are provided.
A dry erase board for messages is also nice. As for modifications, it depends on what you do, but I think its pretty loosely enofrced. The main thing they worry about is fire hazards (so dont have a halogen lamp; they busted me for that or play with matches or be in a room when others are playing with matches; they busted this friend of mine for that) Also dont do/get caught drinking/smoking marijuana in the dorms (penalties are looser for drinking)
Thats about all I remember from my dorm experience, oh and make sure to not lock yourself out of your dorm room or lose your key, lol that happened to me when I was in the shower, had to go to the front desk in a towel.