Memory is not the same as hard drive space. The former is where the PC works with programs and data while the latter is where programs and files are stored all the time. In essence, a computer is able to perform better when it has more memory. There can really be no such thing as too much memory once a computer is used.
The kind of tasks performed and the software type would determine the amount of memory needed by a PC. The memory should match the speed of the PC’s motherboard and this information is easily obtained from the documentation that came with the PC. The PC supplier can also provide the required information for upgrades.