As with most questions about computer specifications, the correct answer is "it depends". A computer with XP and 1 GB is just fine for my parents and sister, but they mostly use it for the Internet, with only a few games such as Civ3, the rest all having similar or slightly higher requirements. And other than a few particularly demanding games such as Civ4 and Company of Heroes, 1 GB would be fine for me since I run XP.
However, I voted 2 GB is the poll since I would assume that this poll would be most relevant in the context of looking for a new computer that most likely comes with Vista. Vista increases what's really adequate by 512 MB or so, and for a new computer it's always better to go slightly higher than what you really need if you can. 2 GB is also a good amount for XP + demanding games. And while some games would do better with > 2 GB, almost every game performs at the very least adequately with 2 GB.
I'd have voted for 1.5 GB if it were an option. But it really depends on the context and operating system.