i'd ask the school and or teacher for spec requirements, see if any are bought in bulk and or svc'd by college dept. when there are simple ills.. Ask about network access, wireless(?) type (A,B,G,N), to make sure that is what sys has (especially if buying used etc.).
i'd stick with XP, they will probably keep selling it until the next os comes out. It is more generally supported than Vista. 64bit is better performance, but; not everything works with it, so i'd stay clear of that too.
i'd consider new with warranty and also damage insurance for non-warranty failures like cracked screen, drink spills etc. (that are accidental damage and not manufacutered failures).
Some systems maid for buisness, don't have the best music playback. i'm sure that is important to a college student. Dell Latitudes and Optiplexes are biz orientated models, and have Lifetime, Transferable, Phone Support, even after hardware warranty fades.