Hm... I had a Celeron machine running XP ~2004-2007 that had 512MB of RAM, and it wasn't hard to run out, eventually upgraded to 768MB but I was still jealous of my friend running XP with 1GB.

Then, I built a Vista machine in 2007 with 2GB to start, and it was clearly not enough, immediately filled the other 2 slots to go to 4GB.