Each upgrade can make a significant difference, and you don't necessarily waste anything, unless you change the graphics card. (If you go from 4GB to 16GB, you waste 2*2GB sodimms.)
Seems to me this was also true of earlier eras, too, only with smaller numbers.
Graphic card needs fast buses, if your mainboard gets behind you won't have access to the latest. I remember struggling with this long ago (pci -> agp; pcie 2x -> 4x).
It may be less an issue nowadays, so my point was off.
ps: It's funny you consider 2011 vintage, my most recent computer being from 2009 and I find it modern :)
Well, that's because I bought my i5-2500K desktop in 2011. I just chucked in a new hard drive and it's about to get another 8GB of RAM ;-)
My previous tower lasted 8 years. I'm not a gamer, but that one got a graphics card upgrade as well....