Often overlooked is radiator caps. They keep the cooling system under pressure. This pressure is important to the overall cooling capability of the system. For every added PSI of pressure the boiling point of the coolant/water mix increases about 2.5 degrees F. If you have an 15 PSI rad cap (hypothetical number - I'm traveling on business so I can't run out and check mine but they usually say somewhere on the cap) you will add 37.5 F to your boiling point. 212+37.5= 249.5
If your rad cap is bad, your system will over heat faster than the same system with a fully functioning cap because your system will boil over at around 212F.