If you are ripping a lot of CD's or DVD's, transcoding video from one format into another, or doing 3D Rendering, you can notice upwards of 30% performance gain.
If you are just word processing, emailing, and surfing the net, then you probably won't notice a thing (unless you can detect milliseconds!).
Personally, I think that thanks to guys like Kilz and his How-To's, setting up a 64-bit system isn't really that much different from the 32 bit system, and keep in mind, you only do the set-up once. After that it is pretty smooth sailing.
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