In theory, yes, overclocking can shorten the lifespan of a computer due to electromigration wearing the CPU's circuits down. Now, this would reduce a CPU's lifespan of, let's say 15 years down to 10. In 10 years, the PC you have now probably isn't going to cut it. As a general rule, if the PC isn't yours, then you have absolutely NO business overclocking it.
You can fry your CPU, although modern motherboards and CPU's have thermal protection of some sort that will slow down the clock speed and/or shut the computer off altogether if a certain temperature is reached. Excessive overvolting can also kill CPU's in short order, regardless of cooling, or if you don't use certain cooling methods (watercooling, phase-change).
Harakim---a lot of what you seem to understand about AMD CPU's is false. AMD's CPU's haven't had problems. They are just as reliable as Intel, and have been for quite some time. Early on, back when the original Slot A Athlon came out, the main chipset, the Via KX133, did have some stability issues, as well as some AGP voltage problems with GeForce 256 (aka GeForce 1) cards. AMD's own 750 chipset wasn't as widely used, but didn't have that many issues, and no major ones that I can recall. I highly suggest that you read up on AMD CPU's, as you don't really have your facts straight as I see from some of your posts.
In general, AMD has a better price/performance ratio than Intel. Now, if Intel would (and I think they will) take the efficiency of their Dothan-core Pentium M and bring a chip out with the FSB's of the Pentium D's, they'd be in the game again. As it stands right now, AMD's performance beats Intel for the same amount of money, in similar dual-core or single-core processors.
This has nothing to do with the quality of the CPU's. AMD may be cheaper, but that does not mean their quality is less. I think Intel can charge a higher premium because they have established their name. However, that's not going to last forever if they can't put out a CPU that's more competitive for the money. I'd use Intel if it wasn't so much more expensive to put together something similar to what I have now with an Athlon 3000+ Socket 939 rig. It comes down to money, and like anyone else, I want the most for my money.