I'm running 8GB ram, however previous system had 2, 4 then 6GB.
This is on XP64, but there is a difference between 4GB and 6GB. I would say 6GB the sweet spot unless I go crazy on the application loading.
I run multiple monitors, and can have 2 visual studios + a couple of SQL management studios, then I may drop into photo editing, or dreamweaver. Then for a break I may load a computer game. I could manage with 6 or even 4, but with 8GB I can go crazy without to much HDD/swap file usage.
For most running single apps 4GB should be fine. There is the advantage if you get 8GB together it's all matched, and for life of system you'll never need to upgrade.