Windows 32-bit and 64-bit and GPT

You've got quite a few different technologies commingled there. I'll try to set out a few facts here:

• Windows understands GPT just fine and is happy to use them regardless of 32-bit vs 64-bit.
• Most BIOS systems do not understand GPT, and thus need a MBR block to boot. Some BIOS systems can boot GPT (I have one at home in fact).
• EFI by spec supports booting from MBR or GPT.
• Many EFI implementations have a BIOS emulation mode, this allows a EFI boot with BIOS compatibility for OSes that do not support booting from EFI. Support for this feature depends on your motherboard.
• Windows 6.1 (7 and 2008R2) support booting from EFI only in 64-bit versions. Some other versions can be made to boot, but this is not recommended at the least.

Implications of the aforementioned:

• Windows 7 32-bit needs to be booted from BIOS, which implies that it must be booted from MBR.
• Windows 7 64-bit can be booted from BIOS/MBR, but can also boot from EFI/GPT.
• Other configurations are possible but are not recommended and may be unstable.

Side note: There are a lot of minor improvements in switching to a 64-bit system. If you have the option always go 64-bit. There's really no excuse for doing otherwise, assuming you have any choice.