I Want To Do Graphics On My Computer. How Much Memory Do I Need?

This really depends on how large your graphics are going to be, and how complex.  The very rough rule of thumb with any kind of photo or image editing is to have as much memory as you can, as long as your operating system and graphics software can make use of it. (There is a 3GB limit on 32-bit Intel systems, and there may be other gotchas.)

Don't forget your processor (CPU) and graphics (GPU), which will also both make the whole thing run smoother and quicker.  A fast hard drive will help, but is not as essential.