You probably won't forget the combination unless you're really that bad with numbers. You'll be putting in the combo every time you want to take your laptop out of the room and the repetition should carve it into your memory.
Keys are good, too. If you live with your parents right now leave the backup key at home so if you lose your main one, you know the backup is in a safe place where it won't get stolen.
Frankly, if someone wants to steal your laptop they're gonna make it happen no matter what. Those locks can be pulled out or cut with wire cutters.
Chances are unless you have a REALLY nice laptop (or a mac), no one's going to try to steal your computer. No matter what kind of lock you go with, it'll do it's job, which is to deter your roomie or his friends from whisking it off your desk. That's the biggest thing in dorms to remember is, even if your roommate's trustworthy, his friends may not be, and you never know who they let in your room when you're gone.
Lojack for laptops is also a good thing to have. Real pros can disable it, but if you notice it's missing immediately, you can call the cops and recover it.