When working at HP I came up with a Windows Vista restore/install solution that worked using Windows PE for both 32bit and 64bit architectures. A few years ago, I tried the same solution against my 2007 White MacBook, and, lo and behold, I was able to install 64bit Windows 7 on my MacBook. I didn't have the time to document it as it was just for curiosity, but I am doing so here now because I am repurposing my MacBook as a 3D Printer server.
To start, you need a 32bit Windows install on DVD and the version of Windows you want to install on a USB key (or another DVD). This is because the MacBook will not DVD boot off of a 64bit install, or the USB key.
1) Boot off the 32bit install DVD.
2) Shift-F10 into a command prompt.
3) Format your hard drive/partition. Make sure you only have 1 partition. You may need to use diskpart, whenever I used the Windows Setup it created 2 partitions which causes 0xc000000f errors on startup because the BCD file is configured to look for [boot]\path\to\wim which doesn't reside on the boot drive.
4) Robocopy everything from the DVD to the HDD. (My DVD drive came up as E:)
Robocopy E:\ C:\ /MIR
5) Insert your Windows x64 USB/DVD and copy over *just* the install.wim from %USB%\sources\install.wim to your C:\sources\install.wim
6) Eject the DVD and reboot your computer off the hard drive. Do the install as usual.
This trick only seems to work when you have both 32bit versions and 64bit versions of the SAME software. It does not appear to work if you try, for instance, to use the Windows 7 Enterprise x86 and Windows 7 Ultimate x64.