I believe that you can order a copy of Vista x64 for just the cost of shipping, and then use your existing product key to install the x64 bit edition instead. That is, however, assuming that you purchased a retail (not OEM) copy, and that you will only be using one of the two at any given time.
Also, note that you cannot 'upgrade' from x86 to x64...you must perform a clean installation, wiping everything and starting with a clean slate. Decide if it's worth it and, if so, backup all of your data first!
Hope this helps,
John
I purchased the 32bit version of vista home premium just to be safe with all the software I already have, but now I want to get the 64bit version of vista. Can I upgrade it from the 32bit, is it a download, or is it even possible? Where can I get the 64 bit version, and how much does it cost compaired to the 32 bit version?
I know my computer is capable of running 64 bit, becuase I just built it, and it is running an AMD Athlon FX-60 processor with 2.5GB of dual channel ddr ram. I also have a GeForce 7600GS video card.
Someone please point me in the right direction, thanks.

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