I would keep it but downclock it a bit for power/noise requirements.
in a few years time how do you know what software and games will be about?
my previous computer lasted 7 years, current one is nearly 5. so going by that, just think, you wont need to buy a new PC until the Japanese Olympics in 2020
unless you really need the hundred or so quid anytime soon that youll get from selling :shrug:
edit: another reason to keep it:
windows/all your software/games are already installed and working. do you want to spend time setting up another rig, when you can just jump on that one whenever and bash out a quick game of something when you feel the need?