I have to agree with Frozenlead -- also, only 1 in 5 people actually use computers.
And, the vote doesn't show those that use multiple 64bit OS's on the same hardware -- I run Mac OS X 10.5.1 and Vista x64 Ultimate on my MacPro with 8GB RAM.
There is little or no point install Vista x64 if you don't have 4GB or more or RAM or don't plan to.
Before those who jump in saying what applications support 64bit, be sure to qualify "applications" -- if you mean Games, not many (a few, very few). If you mean real world get things done applications such as Video editing/composition and/or Audio creation and/or graphics and 3D Processing then you will need as much RAM and processing power as you can get.
I've got Photoshop HDR files that are over 100MB per image -- keeping a good undo level state and my RAM is quickly consumed. My single SD based video capture from a MiniDV device will be 4GB+ file size (just for SD stuff, not even HD quality), the more of this data I can fit into RAM the faster and smoother my video editing experience. And then toss in video compositions (Motion 3) where even moderate projects will require RAM previews (just not enough CPU power even with 8 processors to keep a steady realtime state at 30 fps) and force render copies of segments of the project. And anyone that does significant audio creation using FX and Virtual Instruments will need to bounce the tracks due to limits of the CPU and RAM.
I could NOT imagine trying to do all this with a handicapped 32bit OS that can't address more than about 3.5GB RAM and has a 2GB RAM thread limit (aka Vista 32bit and WinXP 32bit). Actually I can imagine, I've done this on a 32bit OS and the process if PAINFULL and extremely slow!!
Vista x64 didn't feel "smooth" until I tossed 8GB of RAM at it.