I hope it will help
1. You do not need a buy Windows 8 System builder for clean installation on a new PC, you will receive a tool to prepare CD/USB installation media with purchasing Windows 8 upgrade
2. You do not need activated Windows System on an old PC to receive a key and installation tool.
3. You should just to know that you do not have a choice between 32 bit and 64 bit to receive a tool for creation installation CD/USB, it depends what system is installed on your old PC.
4. If you a got a key, your new Windows 8 depends only of installation media you are going to use (32 or 64 bit)
Now how to make a clean installation on new PC using a key received as an upgrade.
1. Prepare you installation media.
2. Your new PC should be connected to internet.
3. Make a clean installation. Choose ” installation with Local Account (not a Microsft account!). Turn Automatic Windows upgrade "off"
4. Check Control Panel/System and Security/System. You will see ” Product Id: Not available”.
5. Restart.
6 Go Settings/Change PC Settings/General/Refresh your PC without...(You will be asked to use installation media again)
7. After that check the activation - it's activated!