This PC, that I am working on, has an 80Gb HDD which is less than 1/3 full after a year and a half, but the speed of the HDD could make a difference, a 7200rpm model having a noticeably shorter search time than a 5800rpm model.
As for your Ram, 512Mg is enough for most everything but Video Editing and the most intensive games. The speed of the RAM could make a difference.
Most increases over what you have will only be the equivalent of putting a pair of tailpipes on your car. It might impress your mates, but it is unlikely to make a noticeable difference to the speed.
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