I started off with Win7 64Bit but went for 32bit in the end. Not only can I use old drivers for old hardware that I have (and for which there aren't 64bit drivers available), but also I can install and run some very old software (from Win3.1 era) without problem (it wouldn't even install under 64bit).
The only real downside of 32bit is that you are limited to accessing around 3.4GB of memory regardless of how much is physically installed. Unless you really need to use more memory 32bit may work for you.
Surprisingly 32bit uses a lot less disk space, Windows Updates are much smaller and I'm guessing that disk transfers will be a shade quicker (less to move around).