It's open to question. Many retailers openly advertise that RAM, CPU's etc. are ideal for overclocking, so it could be argued that they are sold with overclocking as an intended use and therfore your 1st year's warranty with the store you bought them from was valid.
After the first year, it all gets very dubious.
Motherboards are definitely covered for overclocking as any supplied BIOS option must be, by definition, within the limits of the board.
RAM and CPU's are definitely dubious as I don't know of any manufacturer that actively encourages overclocking. They would far rather you bought a higher rated part for more money.
Graphics cards are a definite no-no as far as I know.
As for over-volting RAM, a lot of RAM now has voltage regulation built onto the stick, so even if you give them 2.5V, they still only get 2.3V if that's the maximum the manufacturer has rated them at, hence you see a lot of posts saying things like "I saw no benefit in going from 2.3V to 2.5V".
Bottom line - get Crucial Ballistix - they always take them back and give you a new set - even if (as I did) you admit you probably blew them up yourself

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