Your experience speaks to my HEART!
This is what I do at home for fun and at work.
Keep those old computers running and upgrading if possible.
I love this!
My LAN consists of 6 "frankensteined" computers that I keep upgrading.
I still have 4 of them running Core2Duo's that I picked up for almost nothing.
Still running DDR2 on a few of them as well.
If I had the money, I would definitely upgrade to SSD drives and a GeForce 750 Ti, but that would costs hundreds of $$
I will be upgrading two of them next week to Gigabyte Motherboards, AMD 6-core CPU's and DDR3 1333
Nothing super fast, but much faster than what was in them.
I even resurrected 5 laptops with Pentium's in them as a small portable gaming LAN. Mostly 2D games of course.
Then at work, I keep old Dell servers running and old Dell Optiplex 520's running. I even found a replacement Power Supply for them because of the small form factor. I have soldered many capacitors on these old motherboards too. All in the name of saving money and keeping things consistent. Obviously not in the name of speed.