Vista uses as much as there is available, not all of it though. So if you are just on the desktop staring at the widgets, it will take up a lot. But when you fire up a program, lets say a game, Vista will back off the RAM and let the game use it. An article showed that Vista will take up more RAM if you give it more RAM. I think it was a comparison between 2 and 4 GB.
This should also apply to graphics too. I remember reading that stuff like Aero won't take up that much power, if any from your video card. I'm pretty sure this was said during an interview with AMD. They released a driver that simply limits/stops Aero from eating it up, which would let your games use it all. Meaning less of a performance decrease compared to XP.
This is all of memory, and my memory sucks. I'll try to find the articles if anyone thinks I am wrong, which I might be.