I've replaced my desktop PC-s every 3-4 years, with consistent 4x memory upgrades:
* Had one with 512MB when XP just came out,
* Then one with 2GB for XP still, then installed Vista on it;
* 8GB, for early Windows 7 versions;
* 32GB, from summer, 2013.
My current one, with 32GB is the first PC where I see the memory being truly wasted. I should have gone for 16GB back then.
However, If I were to buy a new PC today, I would get 32GB again for sure, because it is now much cheaper, and I have apps that can use lots of memory.