to tell u...i think u cant.and it's better u dont...cauz even on 512 MB ram. i installed vista and it does run normal..but only for small multimedia and other purposes like surfing...nothin else...4get gamin on vista..untill u have 2 gigs of RAM.
either get more RAM .or dnt install vista...and ur HDD space too will be wasted as it almost takes 7 gb after installation. and u just have 40 gb ..