That really depends on the processor, some processors simply overclock better than others. Your processor is already of age so you might as well bump up the voltage. Any decrease of lifespan shouldn't be of too big of concern, by the time it dies you will very much be overdue for an upgrade.
I'll probably shoot for 3.0ghz. However, I'm not to familiar with the core architecture's limits so I do not know what its overclocking capabilities are.