Your system is old enough that you would probably need a PSU upgrade for something that powerful to work. If your PSU was good enough, you could technically get a 3080, but your CPU will not be up to the task of matching the performance of the 3080. It would be best to do a whole system upgrade at this point, because a new CPU is recommended, and that would require a new CPU, Motherboard, and RAM. Once you replace the CPU, Motherboard, RAM, PSU, and GPU, you have built a whole new system, aside from Case and storage. I would recommend building a new system, and then you still have a complete system that you could use as a server PC, a backup, or sell as a working PC to recover some cash.