Can easily run on 512 MB... As sugested by these guys suerly turn off Aero.. also if u have a good USB flsh drive with you, ten use it for Ready Boost and you will be good to go......
Have he;ped my cousin to install Win 7 on his crappy laptop with 512 MB ram.. But with a USB, its running great.... Really surprised me with the performance...

Give it a try...