Pure ethylene glycol (100% antifreeze, 0% water) has a boiling point of 197 degrees C, water boils at 100 degrees C. If you want to know percentages inbetween check the link below.
http://en.wikipedia.org/wiki/Ethylene_glycol#Antifreeze "Ethylene glycol boiling point vs. concentration in water"
For every PSI of pressure the rad cap adds to the cooling system, ups the boiling point by approx. 3 degrees C.