using disk space as ram is called "swap". Swap on a desktop is slow and should be avoided, but it's usually there just in case you need it. SD, SDHC, and even SDXC cards are even slower than hard drives. If you start using your SD card as swap, it will negatively impact performance.
That being said, as Prometheus said, you aren't likely to need that much RAM. If you do find yourself using too much RAM, we can certainly help you setup some swap space, or at least or figure out why your using so much RAM and perhaps tweak things to use less.