I'm not familiar with your skillset or interests but a bunch of old PCs can be an amazing way to learn from. A palette worth of 90s office pcs pulled from a dumpster is how I ended up learning how to build a linux server in the early 00's. Teach yourself PC building by putting the best parts together in one machine. Learn overclocking, try out linux/bsd/plan9, or create your own webserver. Keep in mind, these boxes are going to consume a lot of power for the performance you get. Don't worry about breaking things. Now's your chance to make mistakes.

The case and fans are good to keep around. You can convert the power supplies to quick and dirty (emphasis here) dc bench supplies with fixed voltages. If they have ide drives that still work, you can use them to mod original xboxes. I will always keep heatsinks for other projects. I usually have a bunch old Dells in my closet to rip parts off of. As you build up a parts collection you rig up stuff for your main PC over time. For example, My main case only had 2 3.5" drive mounts so I unscrewed a 3.5" bay from an old Dell case and rivetted it to my main pc case.