Having a 64 bit operating system does not mean you need more memory, it just enables you to use more. With 2gb, you probably would not see any more performance than if you were using a 32 bit operating system. In terms of having enough, it depends on what you do. If you can afford it, you should get another 2gb stick because it will increase performance in gaming and other areas.
RAM is so cheap now that you can get 4GB of DDR2 800 for $32, I suggest that you do that to improve performance in Vista.