OT a little, I remember going from 256mb to 384mb under Windows NT 4 and thinking that I couldn't really tell the difference.
More on topic, I and likely a lot of others are asking the same question as you. Then there are those who have said Fv<x it and installed 4gb to start with. I don't think that 3GB is overkill in Vista, but you should survey your prime apps and see what you really need case by case. That really should tell you the answer.
This where we are, two GB not always enough. Four GB not utilized fully under 32bit OS, and Driver issues abound for 64bit OS. So I guess 3 GB is hardly the wrong answer.
dung.