I just acquired 13 (approximately 9 of them are actually fully functional) HP Compaq Dc5800 Small Form Factor PCs. I believe it would not be too hard to add 8GB 800Mhz RAM, 750GB HDD, and an RX 550 totaling ~$135. If that's the case, could a Core 2 Duo E8400 + Rx 550 run modern games at low-medium settings 50+ fps? My biggest concern is power, as the included PSU is 240w and they are fairly old PCs. They came from a school district. However, the rx 550 pulls power directly from the motherboard. This is good because I don't need to buy an extra PSU, but I don't know if the motherboard can handle it.
Approximate PC part picker link: https://pcpartpicker.com/list/qCbZyf
Approximate PC part picker link: https://pcpartpicker.com/list/qCbZyf