I'm having trouble using up my 8GB of RAM on Windows 7 64bit with i7-860. It might be because I have prefetch and superfetch off, but with Day of Defeat: Source and GTA:4 running at the same time (they're in windowed mode and I switch from DOD:S to GTA4 when I die) I wasn't even at 5GB of RAM usage.
i don't think the average person will ever use 6gb. i have 2gb and never max it out. sometimes i think people buy more memory than needed only because everyone else is doing it or someone tells them they should. besides, running day of defeat and GTA4 simultaneously, most people are never going to do that for any practical purposes.
i run windows 7 64-bit, too.
I have 4Gig and I have never seen my PC using even 3GB(I'm a gamer BTW)
The max I've seen is 2.7GB used when GTA4 was getting installed
if have have 64bit you are fine, though you may never have an app push you close to that limit
i have 8GB ram and the only time i came close to the limit was a project for school (think infinitely recursive maze, find the best solution, delete an edge and repeat until you can't)