No, you compare 2GB with 4GB and you will notice a difference.
Ok, if you are only doing one thing or your are not using the full power of your PC then maybe 2GB might be fine for you, but if you are going to use your PC properly, then 4GB is a must for Vista.
I have one Vista32Prem Setup upstairs and thats got 2GB on it, but then, thats only ever used for my 10 year old daughter to play Little Mermaid etc on... I would never dare be as nasty as to drop her RAM to 1GB - Im not that spiteful, but I also see no need to up her RAM to 4GB either.
On the other side of the coin, my 9 year old son is running Vista64 and he has 4GB... Originally, he too had only 2GB and his PC ran just fine on that, until I ran a few tests and found the extra 2GB really did make a bigger difference, so for my 9 year old son, I would never be cruel and force him to only use 2GB on a vista machine again. No, its not vital but when you compare them, then, providing you are using your PC properly, then you too will find 4GB to be better than 2GB - even though you think that 2GB is fine now.
I myself have now opted for 8GB in my main box and 4GB in my No2 box and the only reason Im still on 4GB on PC2 is that the Mobo wont do more.