To be honest, there are very few upgrades for that Mac that are condoned by Apple. The internal hdd could be upgraded, and the RAM, I think. CPU and GPU not beyond what may have shipped in some variant of custom order for that model year. One could probably upgrade the optical drive to a faster model. And of course the OS.
But by today's standards it's definitely outlived its usefulness for the average user. It's not a bad hobby machine, though, if you only just want to tinker or keep it around for sentimental reasons. I've let a lot of old Macs go myself because the aggravation wasn't worth my effort.
Best of luck. 🍀