click here is a sort of summary.
In a way you have offered the answer yourself. Checking around (try all your hardware like printers for example) shows that 64bit drivers are available in limited supply. The key point is even if they are there 64bit Vista is set up to only accept signed drivers. Some software is not put through the MS testing process and therefore is not signed.
The max RAM for 32bit is 4Gigs but that is for all mapped RAM requirements so a graphics card with say 256 Mb of RAM will take part of the 4Gb allowance. Typically therefore 32bit sees about 3.4 Gb of usable RAM.