It will use only about 3.25GBs out of the 4GBs now anyway and the rest is reserved for hardware, etc
@@@@@@@@@@@@@@@@@@@@@@@
32bit > 64bit is not done by any upgrade.
It requires a clean install of the operating system.
Save all your data and be prepared to reinstall all your 3rd party programs.
Then do a clean install:
http://www.sevenforums.com/tutorials/1649-clean-install-windows-7-a.html
_______________________________________________
But before you attempt it, check with your computer/laptop manufacturer to make sure that they support and supply 64bit drivers for your machine and for the edition of Windows 7 you are licensed for.
Also check your computer specs to see if you have or can upgrade to at least 4GBs of RAM, as without at least 4GBs of RAM you are wasting your time installing a 64bit version.
Or it may be easier for you to download the Crucial Scanner which will tell you what is the matching RAM for your Motherboard and the max. amount of RAM your Motherboard can accommodate.
Crucial System Scanner
http://www.crucial.com/usa/en/systemscanner
@@@@@@@@@@@@@@@@@@@@@@@@@
You don't tell us what type of license you have: Retail, OEM preinstalled or OEM System Builders license.
If it was Retail you don't need to buy another license as a Retail covers 32/64 bit but with only one installed at a time.
@@@@@@@@@@@@@@@@@@@@@@@@@@
You take your chances no matter where you buy 7 now.
Microsoft no longer sells it; try amazon.com.
Warning:
You do not buy a Product Key by itself, as they are normally stolen/pirated, Volume Licenses which are not for private
use, or MSDN keys which are not to be sold by the original subscription holder.
Make sure that you buy the complete sealed package (DVD, license)
https://www.microsoft.com/en-us/howtotell/Shop.aspx
Read Section: Beware of Product Keys sold separately.