Nope, it uses the 1 Gig just sitting there! I may have had my mail and IE7 open but nothing more! 
That said Vista prolly isn't using it all, there are things in the background like my AV and such, you have to remember though that Vista has lots going on in the background, like all the checks it constantly does, Defender and other stuff running.
You can get quite a lot back by turning off some of the bells and whistles, there's lots of stuff you don't need.
Don't forget though, I've only been playing with it for 4 days so I might not be using it effectively at the moment!
If you search on various tech sites though, they pretty much say the same, it runs ok on 512 but really needs 1 Gig!

Injustice will be avenged.
Cofiwch Dryweryn.