Should I upgrade my GPU or build a new PC?

I'd replace the GPU and keep the rest for now. The 12400F is still a capable CPU, and there's no obvious reason to replace 64GB of DDR4 if it's working properly.

Pick a card based on the games, resolution, and frame rates you want rather than buying the fastest one available. You can also move the new GPU into a future build if you decide to replace the platform later.

Check the power supply before ordering a card. Look at its wattage and available GPU power connectors, then compare those with the card manufacturer's requirements. A prebuilt may have a modest power supply or limited connectors, so a stronger GPU could mean replacing the PSU too.

Also check whether the prebuilt uses standard power connections before buying a replacement PSU. You don't want to discover a compatibility problem halfway through the swap.

You don't automatically need a new case just because it's a prebuilt. Measure the space for the GPU and compare it with the card's length and thickness. Check airflow and PSU fit too if you're changing the power supply.

If the case is cramped or uses nonstandard mounting, a case swap might make sense. It won't make the PC faster by itself, though.

First decide whether you actually want more performance or just want the machine working again. A modest replacement card could be enough if the 3050 was already doing what you needed.

A used GPU is another option if the price fits your budget, though you'll want to check its condition and the seller's return terms.

A full rebuild isn't off the table, but the RAM cost depends on what you actually need. You could compare the cost of a new platform against a GPU and any necessary PSU or case changes. If 64GB is more than you use, selling some RAM could offset part of an upgrade, though resale prices will vary.