Windows 7 x32 or x64

Hi everybody,
my hardware is capable of running both versions of 7
I would like to order "Microsoft Windows 7 Professional Upgrade" available as download from MS website.
I would not like to commit myself to 64bit version of 7 and in case of disappointment go for 32bit.

Will the key from x64 "upgrade" work on x32 "upgrade" and the other way around?

The physical retail Upgrade DVD comes with 2 discs, 32-bit and a 64-bit.

Hope somebody can help me. I've just bought a new computer with Win 7 Pro 64-bit installed. I have the OEM disk (64-bit).

I now find that W-7/Vista 64-bit drivers are not available for some of my hardware (in particular my Dell 2405FPW monitor and Philips Voip321 Dualphone). I want to "downgrade" to the 32-bit version.

I have a 32-bit disk available and if I understand correctly from reading other posts my 64-bit Licence Key# can be used for either version. Is this correct?

My next question: can I install the 32-bit version "over" the 64-bit version or do I have to do another clean install and re-install all my other software?

One more question: if I change my mind again, and want to revert to the 64-bit version, can I reverse the process ... i.e. install the 64-bit version over the 32-bit version?

I'll be grateful for some expert advice.