If you have a G3 850, you certainly don't need a power supply. That's within a few models of about the best you can get, maybe THE best for the money at that capacity.
There is probably nothing you need, however, my thought would be maybe it would be a good idea to get a 4TB external hard drive to back up everything that is on all the other drives, since you likely have different information on each drive if you are like most people, without any of it backed up to safe extra location. Even if one or more of those drives is being used to backup the others, it would STILL be a good idea to have an external backup of the backup, because I've absolutely seen ten or more instances where somebody zapped all the drives attached to the motherboard at one time. Some through sheer stupidity but others from power surges or strikes. Never hurts to have a good backup drive so you don't lose any critical files or documents, or game installers and so you can make full drive (All partitions) backups of your OS drive periodically in case something gets corrupted or there is a drive failure.
Other than than man, the only worthy upgrades I see are an M.2 drive for the OS, and I'd recommend the 970 EVO+ or 970 Pro M.2 PCIe NVME drives, or an upgrade to the flagship graphics card.
Maybe a better monitor? What kind of monitor, or monitors, are you rocking?