I don't know if it's this simple but:
If your computer is for gaming, play games you like at settings that you want, and see if CPU, GPU, or memory utilization reach 100%. Whichever it is, that would be your bottleneck.
Edit: while your CPU is getting a little older, your RAM is also pretty slow. It's hard to say if you need to upgrade the RAM alone. Because if you do, you might as well upgrade the CPU (and motherboard) at the same time.
I feel the need to be informed of his rantings and actions, but I also find it nauseating. Any advice?