Basic usage of Virtual memory:
The OS looks at the RAM for anything that hasn't been used recently and copies the data to the hard disc area allocated to virtual memory. This frees up space in RAM to run more applications/processes.
As HDD RW speeds are slower than RAM, if your PC has to rely heavily on virtual memory, you will get a significant performance drop.
The default level of virtual memory the OS sets is generally fine.
It would be best to get more physical RAM if you are worried about memory performance.
