Upgrading RAM and computer speed for video editing?

I have an old computer( Intel Pentium 4 with 1GB of RAM and 1.60 GHz), and I want to watch and edit my HD video projects without dealing with the slowness of an obsolete computer. I'm wondering how I can add RAM speed and processing power to the computer.OR, if needed, can I keep my hard drive with everything from my computer, and move it to a faster one with a better processor. Suggestions?