jyjjy :
Yes, overclocking is extremely safe these days. Current chips have an almost excessive amount of safe guards making it nearly impossible to damage your system even if you were to purposely set out to do so. I don't believe it should void the warranty either and even if it did they will only know you OCed if you tell them.
Changing the speed or voltages of your CPU, RAM, or GPU instantly voids both the manufacturer and any store warranty the very moment you do so.
I do not know of a single hardware manufacturer, or PC store whom will allow you to keep your warranty on a component if it has been overclocked in any way. If it breaks, you have to pay big $$$ for the repair.
Most pre-built PC's, especially from Dell or Gateway, have overclocking features permanently disabled in the BIOS by the manufacturer, even if the motherboard and BIOS naturally support overclocking. Such as my current Gateway FX-6710-01 desktop has.
I bought a ZALMAN CNPS9900 because the stock Intel cooler the Core i7 930 came with is notorious for being an inefficient piece of junk, and because it was on sale for $40. Processors run extremely hot using it, even at their stock speeds. My CNPS9900 cooler isn't the best, but it's sure a heck of a lot better than the stock Intel heat sink and fan. Most of the reviews I've read for it have been widely positive, it should also give me a bit of overclocking head room when I feel good and ready.