I have 1 GB on my Desktop and using Windows XP Home Edition On the laptop, I just upgraded to 2 GB and using Windows Vista Home Premium
1G ram Windows Vista. Yes, It is a litle short on RAM but it works, and no complaints on heavy software.
I have one windows XP machine and one Vista Ultimate each running 3Gb. 3Gb seem to be the most effective amount of RAM for 32bit machines. I sometimes run a 4Gb USB drive with readyboost, but don't notice a difference.