photonboy :
I still recommend 8GB (2x4GB) for most users despite the guy who "hates that mentality" above somewhere. In Canada it's $80 currently for another 8GB. By the time you benefit from more than 8GB you'll need a new PC.
At the rate CPU performance is progressing, I think most people who do not let their inner geek overrule rationality will not feel much of a need to upgrade their CPU within the next 3-4 years and by then, they may very well wish they had upgraded to 16GB while DDR3 was still cheap to get another two years out of their system.
Back when I built my current PC, DDR3 prices were close to their all-time low and since I am used to paying twice as much for RAM than the CPU, I did not hesitate much to simply get 32GB for $180 while I could - I have always enjoyed how much smoother PCs feel when all frequently accessed files can fit in the OS' disk cache.