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.