As an old school doom mapper from the 90s, it absolutely isn’t. You build a room, apply F_SKY1 to the ceiling, and that’s it. The engine renders it more like a wall, but it’s not a traditional skybox in that it doesn’t exist in the level geometry outside of the room it’s applied to.
Any new parts are most likely going to necessitate a complete rebuild. Your mainboard probably tops out at PCI3, a new GPU will be PCI4. It’ll work, but you won’t see as big an improvement as you expect. Any new CPU is going to need a new mainboard because intel. New mainboard means upgrading to DDR5 RAM because that’s the standard now.
Alternatively go the secondhand route, you should be able to pick up an i5 or i7 that’s socket compatible for not much.