Meh, I'm the overclocking guy, and I don't suggest overclocking Dells.... They're budget for a reason.
But overclocking it right WILL NOT reduce the lifespan of the computer. IF you do it right. Even if it does reduce the lifespan, even by half, your computer is designed to last longer than you'd want to use it anyway, and halving it would not change that fact. Unless you're thinking of handing your computer to your grandsons/granddaughters, you wouldn't notice any reduction in lifespan.
And we're just talking theoretical lifespans here, just like the theoretical lifespan of a HDD is 5 years. Use that information as you will, I understand that there are some HDDs which last 6 months, and others, forever.
In either case, your Dell just isn't built with overclocking in mind. Half the parts are custom to Dell, and most Dell BIOSes are locked from overclocking anyway. Your PSU is a cheap piece of crap, which even limits your graphics card upgrades.... Sometimes you can overload the PSU just by plugging in more HDDs, what can I say?
And the other thing is: why would you want to overclock? I'm talking as if there are no risks, but there still are risks, even when you do it all right. The chance of something going wrong is minimal, but its there. If you're not going to see any difference in your daily computing, its not worth it. If you game, it might help some. If you do some CPU intensive work, you probably wouldn't have bought a Dell.