You may change your Windows 7 OS to a 32 bit system, but to be honest with you there is really no reason to do that.
Windows 7 is far superior to Vista and I believe you'll find that virtually any 32 bit software will run fine on it in a 64 bit version.
64 bit is here and now, AMD and Intel are only making 64 bit processors now with the exception of the Intel Atom which is a 32 bit for netbooks. I don't believe AMD even makes a 32 bit CPU.
Try the 64 bit first and see how you like it. You'll need a minimum of 2GB of RAM per MS recommendations, but most of us running it go with 4GB of RAM.
You can always install 32 bit version later because your product key from MS is good for both versions of whichever edition of Windows 7 you decide to go with, plus we have links here to download the 32 bit ISO image files of Home Premium or Professional. You'll find it as a "sticky" in the Install and Setup section of the forum Home Page.
I hope this helps to answer a few of your questions and concerns.
Remember though, with your product key you are only able to run just one version of Windows 7, either 32 bit or 64 bit, not both.
OH... Welcome to the Windows 7 Forums website! 