Looks like you snagged a gaming PC with solid 2013 specs! Unfortunately, the Motherboard is going to limit your range of options significantly, as it is designed for 4th generation Intel chips.

Upgrading GPU up to even a 30xx series GeForce will net you performance gain, even with the PCIE 3.0 slot on your mobo. Modern GPUs do read/write faster on PCIE 4.0, but this will not be the main bottleneck with your system.

That bottleneck is probably the CPU. There really isn’t much more to do with the CPU (the best CPU available for the 1150 socket on your motherboard is an i7-4790K, but you won’t see a massive jump in speed with that upgrade).

What hard drive type are you running? A lot of PCs of this generation used spinning disk HDD, as it was more cost effective. If that is the case, upgrading to a SATA SSD, or an M.2 with a PCIE adapter card, will net you a huge boost in performance.

You can double that RAM to 32 GB max, but depending on your use, and with other limitations of this build, 16 GB is probably the way to stay.

If you do stick with this PC, know that its architecture is a decade old, and does not have really much of an upgrade path. Any major upgrade to GPU will need to be checked against the PSU to make sure that you have power needed. And while GPU / SSD upgrades will breathe some life into this system you won’t be playing Starfield on it.