Huh. Difficult question; depends what you're using for reference. I would have to say motherboards get outdated (and so, depreciate) faster than memory or chips. Of the three, processors will probably depreciate the slowest (e.g. the 1.4 GHz Athlon is still going for $80 on PriceWatch even though it's pretty old... I bought one in July 2001 for $160, so it took 2 years to drop 50%).
Of course, that's all IMHO. Markets can change, certain technologies can surge ahead of others, etc., so no one can really predict very well.