going to 450-500fsb usually requires a bit of extra voltage on the mch or whatever, and that would likely decrease the lifespan of the motherboard, but that effect would be lessened if your cooling around the motherboard was sufficient for those extra voltages. if you were doing something like folding @home 24/7 on 450-500fsb without great cooling on the board for example, that might really shorten its lifespan.