Old School Chores No More: Memory, Hard Disk upgrades

For a long time I have not heard late 1980’s tips like adding memory to your PC, upgrading your hard disk and other tips one would read in the editions of PC World and PC Magazine.
However, someone asked this in the NY Times Circuits I need to add memory to my computer, but am confused by all the numbers and labels, and I worry that if I get the wrong kind, it won’t work. How can I tell which type to buy?.
It got me thinking, as computer prices are now so low and are computers are so powerful, there is less and less need to upgrade. I wonder if hard disk and memory vendors are suffering just a bit as we are upgraing our computers less?