I'd still say 4GB is adequate for the majority of users to be honest, but for my next build I'd feel inclined to stick 8GB in it, it's not that common to be bottlenecked by RAM on a new build, 16GB is still a huge amount in comparison to what is required day-to-day
People don't really want to pay much money for an older machine, I don't doubt that back when the PC was built it would have been a beast, and would probably still serve you well for most games below 1080p, but the lack of support for later Direct X and OpenGL would let you down eventually I'm sure.
Stick a modern mid-range graphics card in it and you'll be laughing!