with the same key"
No, you can't. You'll buy either a 32bit version or a 64bit version, and although both versions will be on your DVD you'll only be able to activate the version you ordered/paid for - you can't just choose on the day. There's no reason why the 64bit version should be a disaster, it's exactly the same as the 32bit version.
If you install a 64bit operating system and you want to run 32bit software your machine will oblige - it will operate as a 32bit system. As far as I'm aware there should be no problems with 32bit games.
You'll need to check that there are 64bit drivers for all your hardware components/peripherals before you start the install. Windows 7 may have them onboard, or it may download them during the install routine if they were released after Windows 7 went to manufacture, but don't rely on it - check with the manufacturers and download 64bit drivers yourself.