No, don't wear a tie. Just be clean and presentable, and try to look like someone they'd want to have around in the fall.
So like, some nice pants and a polo/casual button down?
Sure. You want to look like a grad student, but less broke and exhausted. Remember the admission director's name, and maybe the registrar's as well, since these are people you will be talking to again and again. Often a current student will lead your tour or give you a q&a session - try to get these guys alone and ask them the real questions, cause they will tell you the real info on the place.