Bought 32-bit, need 64-bit? The End User License Agreement for Windows 7 entitles you to activate either the 32-bit or 64-bit configuration of the edition you bought on one PC. Problem is, system builder DVDs come with only 1 configuration. Although Microsoft doesn't talk much about it, they make available the resources for obtaining the 64-bit configuration which you can activate with the product key that came with the 32-bit.
All you need is a blank DVD (because you most likely have access to a PC with a DVD burner [also called a SuperDrive, NOT a Combo drive]), a fast internet connection (you're going to be downloading at least 3GB of data) and a simple utility that lets you select Windows 7 Home Basic, Home Premium, Professional or Ultimate from a single .iso image.
The instructions are here:
The DVD you create is the same type a PC tech would use to install Windows 7 64-bit on a customer's PC running Windows 7 32-bit. The new installation wipes out the old one, and that's what keeps things legal. I used the homemade 64-bit DVD on my MacBook Pro and activated the operating system with the product key from the 32-bit DVD I had bought from Amazon.com. If Microsoft's activation server had detected any monkey business, the activation would have failed. The installation also passes Microsoft's Genuine Windows validation.