you pick ram pretty much on what OS and OS version your planning to use.
but even vista 64bit. i would never need more then 4gigs.
even high end games wont ever need more then 3g at any 1 time. so its about the multi tasking you need.
and even then, if i want my 64 ultimate to run a battlefield, while having a taskbar full messengers, and a firefox with 16 tabs open all day. and the sidebar with hardware monitoring apps and news feeds.
i STILL dont use more then 4g