I recently migrated from XP to win7 x64. I have an i7 920 with 3GB of DDR3, almost everything runs better in win7 than it did in XP and it uses up between 800 and 1000MB of RAM in idle. That's right, win7 x64 uses up less than a gig on my system.
As for why i have 3GB of RAM, when i bought my system a couple of years ago there was no win7 yet. Having worked with Vista while serving my army/conscription term i wasn't going to touch them with a 10 foot pole (there's nothing worse than Vista acting up when an air force brigadier with a couple thousand flying hours in mirage 2000s is waiting for his weekly briefing slides
).
As for the alternatives, XP 32 bit couldn't "see" more than a total of 4GB of system RAM+GPU RAM due to address space limitations (XP 64 was out of the question due to driver issues) and i already had a GPU with 1GB of RAM, plus the i7 and mobo have that triple channel capability, so i ended up getting 3 sticks of 1GB each.
Back to idle memory usage, maybe it depends on the amount of total RAM, who knows? Maybe the OS reserves more RAM for system tasks if it detects you have some to spare, could that be possible?