I know from experience that the retail version of Windows comes with a 32-bit DVD and a 64-bit one, and that the product key is good for both versions.
However, as ignatzatsonic rightly points out, you can only install and use one or the other at the same time, not both. http://windows.microsoft.com/en-us/w...sked-questions
As also suggested, do check first to see if all your hardware is capable of being used in a 64-bit environment, although I have to say, also from experience, that I haven't found a great deal of difference in performance between either version when using 4 GB of RAM. http://www.microsoft.com/download/en/details.aspx?id=20
It's not so much the age of a driver
that gives cause for concern, it's whether 64-bit drivers
have been written for it. Of course, it's also very true that older hardware is less likely to have 64-bit drivers, mainly because not so many people were using 64-bit systems because they just weren't available.
You may also have to reactivate, but this is easily and quickly done by a robo-call to Microsoft's activation server.
Do also be aware that as the license is good for one computer only, you won't be able to transfer it to another computer, in the same way as you can't transfer an OEM license.