You will have to do a clean install. You cannot do an in-place upgrade from 32-bit to 64-bit.
You'll need to backup your user profiles using Easy Windows Transfer to a separate drive (or DVD or some other backup medium), format the drive, install 64-bit Windows 7, restore the user profiles using Easy Windows Transfer, and then reinstall all your programs/games. There is no way do to an in-place upgrade.
Also, you better first check and make sure all the hardware you currently have has 64-bit drivers. If they don't have 64-bit Windows 7 drivers, check to see if they have 64-bit Vista drivers. If so, then you're good to go, otherwise, your hardware may not work under 64-bit Windows.