Vista takes up about a gig of ram from what I have observed.. Also I've read some stuff that suggests the way it manages your RAM is different as well.. For whatever reason it may appear to be gobbling up a lot of ram, but when the ram is needed it releases it to be used or something along those lines.
Right now I've got 1.1GB used roughly, and 2.9GB free, out of 4GB. This is according to a sidebar app for my desktop. Going into task manager it claims to be taking up all but less than 1MB... Don't have any performance issues or anything either so whatever it does isn't hurting.