Where are you getting this information?? I've upgraded multiple XP machines from 1 to 2 GB, and the performance boost is impossible to not notice. Not just in games - windows starts faster and every app loads faster. I've also, for my own tests, wiped an xp machine with 1 gig, installed windows, ran it for a little bit, installed 2nd gig, and again, the speed improvements were immense.. I haven't seen or heard anyone who worked with computers that would recommend less than 2 gigs on any windows machine for any use.
Anyone who still has 1 gig of ram, please ignore this guy, go buy another stick.. it will be the best 20 bucks you ever spent on your computer.
And no, 512mb of ram is not any kind of stopping point, or good enough point on a cell phone.. In fact, I have a feeling phones are soon going to need more RAM than computers... they have cameras, GPS, 3g/4g to manage too.