I am not sure about Vista 32-bit. But my Vista 64-bit takes more than 1 GB right from the start. Most of the time it consumes about 1,5-2,0 GB only for Vista.

Yesterday I checked the RPG "Witcher" and it consumed just like QuakeWars and ZBrush only 512 MB RAM. So Vista is really hungry in terms of hardware compared to modern games.

So I recommend to use at least 4 GB for Vista 64-bit.
I am sure there are some other 64-bit OS out there with less hardware consumption. But I need the Windows compatibility.

And because some of my programming tools raised problems on Vista I just installed Virtual Box to install XP in a virtual environment. And XP is able to boot in this virtual PC in 5 seconds, really, no exaggeration. This is amazing.

Since I have 2 monitors I can run XP on one and Vista on the other one and can work parallel in both systems with only one PC. This is great and I have maximum compatibility.