Well, I’ve managed to get as far as booting from a USB Flash drive into Win7 64-bit, but it’s falling over after the bootloader.
To get this far, I used method 6 from http://www.bootdisk.com/pendrive.htm (why isn’t WordPress letting me add links?) to build the Boot USB – using my existing 32-bit Win7 installation. I then copied the entire contents of the Win7-64 ISO onto this USB.
Taking tips from other sites about booting USB drives on Gigabyte boards, I then inserted the USB drive into the rear USB slots (not wire-connected ones at the front), and booted into the BIOS. Going to Advanced Settings, HDD Boot Order, I can see my 3 conventional HDDs, and the HDD Flash at the bottom. I moved it to the top.
I suspect the f12 Boot Menu has no impact if you select USB-HDD, USB-CDROM, or any other; you actually have to go into the ‘conventional’ HDD list in the BIOS with the USB inserted, find the USB drive, and move it to the top.
Anyway – it now boots from USB, but bluescreens with an error about a file in \windows\system32. Since I’m booting from USB, which doesn’t have such a directory, I suspect it’s either going back to my fixed HDDs and getting confused, or its building a RAM disk and falling over there. My next step will be to unplug the DVD drive in case it’s still causing problems, and then maybe move onto playing around with Win7 Boot loader sequences.