32 bit can only use 3.2 GB of RAM, no matter how much is installed. 64 bit OS's will be able to use whatever you cram in there. There are some other differences that, at this point, won't make a ton of difference to you. Some software (iTunes for example) requires a special 64 bit version. You can upgrade, but I would do a clean install. You can do this with the upgrade version, just google how to do it. You should have a 32 bit and 64 bit license with your Windows 7.