If you really want to be preventative, change the entire cooling system. The plastic hose connectors on the radiator will fail much the same way the expansion tank will, hoses will start cracking and so on. The plastic parts of the cooling system seem to crack and fail anytime afer 60k.

Do you have a V8? I had my valley pan gasket changed the same time as the water pump.

__________________
Profeshenal spellar