Old pcs won't make modern gaming rigs, and if you decide to upgrade them to the best available parts, no shock that is often the goal of fans of old hardware so prices are usually insane. I have an old media player, essentially a laptop made into a tiny desktop tower, which also uses a dual core. Best CPU and ram it can use is iirc a T9500 and 4 or maybe 8 GB SoDimms, currently about $150 for cpu and 2 sodimms. So far no luck on any reasonable priced ram above stock size, but I did find a T9300 which is better than stock by a fair amount, and not much worse than the T9500, and it was about $12.
Old pcs are fine for learning, get a few of the same era and socket type and fix them and mix the parts around to see what does and doesn't work.
For a working decent gaming rig, at least starting out building pcs, best to strictly follow a known good recipe speccing all the parts etc.