It really depends on the applications you plan to use. For basic use on a Windows 7 or Windows 8 computer, 4 - 6 GB is adequate. If you intend on doing more serious gaming or applications that require quite a bit of memory, then 8 - 16 GB tends to be sort of a "sweet spot" for more enthusiast types. It's only when you get into professional software that more than 16 GB becomes a desideratum.
It really depends on the applications you plan to use. For basic use on a Windows 7 or Windows 8 computer, 4 - 6 GB is adequate. If you intend on doing more serious gaming or applications that require quite a bit of memory, then 8 - 16 GB tends to be sort of a "sweet spot" for more enthusiast types. It's only when you get into professional software that more than 16 GB becomes a desideratum.