I think sometime in the first half of next year for people who seriously use computers. I just upgraded yesterday from 1GB to 2GB on my home machine. While there was maybe some small performance boosts that I barely notice, the only big ones so far are switching out of games (alt-tab) and exiting to Windows after playing game. I?ve also noticed Firefox is more responsive after doing nothing in the taskbar for hours. At my work (Software Development) I know I could use 2GB of RAM now. However, for people like mom, it will be several years before she needs 2GB since 512MB is more then enough for now.
So my prediction is by next June the recommendation, around here (Anandtech), will be 2GB.