I thought this video was rather interesting, because at 12:27, the presenter crunches the numbers to find out how many years it would take for a new computer purchase to be more environmentally friendly (in regards to total CO2 expended) compared to using a less efficient used model.
Depending on the specific use case, it could take as little as 3 years to breakeven in terms of CO2 if both systems were at max power draw forever, and as long as 30 if the systems are mostly at idle.
I have a 4-node heater here, but only 2 nodes are in use currently. I got it because it was exceedingly cheap (£75 here in UK and all 4 nodes have 3xE5-2620s and 48 gig ram) but in reality it overkill. Tempted to make a solar powered rpi 5 + m.2 server with battery backup just because I can, but it will be for serving websites both static and wordpress