3Year warranty on this spec PC for �50? Is it worth it? And should I overclock it?

Extended warranties are one of the most unpunished scams in the computer industry. If a computer is operated normally (ie: not overclocked), then computers will, in theory last almost forever. They will at least last long enough to become obsolete, which is the fate of most computers today. If you overclock the machine, it will void your warranty and yes, they will know you did it when you fry the processor.